Abstract: Based on the relationship between sap flow and transpiration, the stem sap flow in different DBH(diameter at breast height)of Picea obovatas on both the sunny and the shady side were monitored using TDP(thermal dissipation method)constantly in the Kanas National Nature Reserve, northwest of Xinjiang. The purpose of this work was to evaluate role of tree size in forest hydrological cycle. The results showed that the changes of the stem sap flow of P. obovatas were multi-peak curves from June to September. Both the starting and peaking time of the stem sap flow of sunny side were consistent with that of the shady side in two DBH class of P. obovatas. The value of stem sap flow in August was highest, and the maximum flow rate in the sunny side of large and P. obovatas of small DBH were 1.45 and 1.19 times of that of shaded side, respectively. The total sap flow flux of P. obovatas of large DBH was 5 974.90 kg, and small one was 1 628.71 kg. On sunny days, the change of stem sap flow was multi-peak curve, but was unimodal type on cloudy days. The sap flow of P. obovatas of small DBH at night persisted longer time path than that large one. The sap flow at night was mainly used for replenishing water on sunny or cloudy days. The mean overnight flow rate of sunny side and shady side of P. obovatas of large DBH on cloudy day were 1.49 and 1.23 times of its on sunny day, and the P. obovatas of small DBH were 1.13 and 1.20 times, respectively. The main environmental factors affected stem sap flow were air temperature, vapor pressure, photosynthetically active radiation, soil temperature, soil humidity and wind speed.