Abstract: 【Objective】Paeonia ostii is a shrub that is native to China and planted widely in the southern and northern regions of the country. The seed of P. ostii is abundant in monounsaturated fatty acids, including α-linolenic acid(ALA), a necessary fatty acid that the human body cannot produce. Thus, the seed of P. ostii is an excellent raw material for a new type of edible oil. There are nearly 20 species of Paeonia plants, most of which are native to China. Due to its large-sized flower, many Paeonia species are cultivated as ornamental plants. Most Paeonia plants have poor fecundation ability. Compared with other Paeonia plants, the fruit-set value of P. ostii is relatively high, which provides a possibility for its large-scale seed production. However, the breeding system and pollination biology of the plant still remain unclear. 【Method】This study examined the breeding system and pollination biology of P. ostii, through its floral biology, pollen viability, stigma receptivity, breeding system and major pollinators. The life of a single flower and the population were observed. Pollen viability was tested with the TTC staining method, in which red pollen was regarded as viable while black and pink pollen were regarded as inviable. Stigma receptivity was determined with benzidine and hydrogen peroxide, which allowed the identification of air bubbles and produced a range of color intensities, with darker colors representing stronger stigma receptivity. The breeding system was detected by applying an out-crossing index, a pollen ovule ratio(P/O), and artificial pollination using emasculation, pollination and bagging. The major pollinator species and their foraging behavior in the field were also observed. 【Result】① The anthesis of a single flower and population of P. ostii was 4 and 17-19 d, respectively. The blooming period of the population showed a mass flowering pattern. During flower opening, the filaments became elongated, loosened and moved away from the stigma, which created a relative spatial separation between the stamen and stigma. The surface of the pollen was dry and smooth. Many pollen grains fell on a floral disc when shaking the flower. The surface of the stigma was wet and sticky. ② The flower of P. ostii was protandrous. Anthers began to release pollen after the petals were completely open. Pollen was released from the top(on the 1^st morning)to the bottom(by noon the 2^nd day)during anthesis. The life span of the pollen grain was four days. Pollen viability was the highest on the 1^st day [(65.2±2.6)%] and showed a weak decreasing trend on the 2^nd day [(57.4±2.5)%]. Pollen viability decreased sharply on the 3^rd [(43.9±1.8)%] and 4^th days [(19.5±2.2)%]. The color of anther locules was yellow on the 1^st and 2^nd days, dark yellow on the 3^rd day, and completely brown on the 4^th day. The filament wilted and began falling off gradually by the evening of the 4^th day. The stigma receptivity was strong on the 1^st day and peaked during the 2^nd to 3^rd days. The surface of the stigma gradually became dry and the stigmatic receptivity had a decreasing trend on the 4^th and 5^th days. The stigma then gradually lost receptivity, until it had basically disappeared on the 8^th day. During flowering, the stigma stayed close to the style before opening, expanded after opening, and became receptive gradually. Mucus could be observed on the surface of the stigma on the 2^nd day. Mucus flew from the surface to the bottom of the stigma and could condense into a transparent gel on the 4^th afternoon. Thus, the expression of male function(pollen viability)and female function(stigma receptivity)overlapped. ③ Numerically, a flower produced(211 ± 10)stamens, 159.7×10⁴-576.3 × 10⁴ pollen grains, and(67±3)ovules. The pollen ovule ratio was 58:940. The out-crossing index was 4, due to the large flower diameter[(15.3 ± 0.2)cm] and relative separation between the male and female functions. Through emasculation, bagging and the artificial pollination test, we found that the fruit set and seeds per fruit resulting from xenogamy pollination were significantly higher than that from selfing. No apomixis, wind pollination, or automatic self-pollination were found. No significant differences were found in the fruit set and seeds per fruit between open and xenogamy pollinations. ④ The flower of P. ostii is nectarless and the only reward for visitors was pollen grain. Each plant could produce( 3.5 ± 0.1)flowers(ranging from 2 to 5 flowers). Thus, a plant displayed an average of 0.8 flowers each day. Petals could keep fresh after the filament wilted, which enlarged the floral display. The floral morphology of P. ostii was not specialized and could be visited by different types of insects. Visitors to the flower of P. ostii included bees, ants, beetles and flies, of which Apis cerana, A. mellifera and Bombus flavescens were the most abundant pollinators. The bees foraged pollen and flew quickly among different flowers, which implied a quick pollen transfer. The quantity of beetles was also relatively high, and they could carry a large amount of pollen grain due to their dense short hair. However, the beetles often foraged on the flower for up to several hours and were thought to affiliate self-pollination of the plant. Thus, the bees were the major pollinators of the flower of P. ostii. 【Conclusion】The breeding system of P. ostii is dominated by out-crossing, with partial self-crossing, and the pollination process requires pollinators. The major pollinators included honey and bumble bees.