【Objective】Populus deltoides is one of the largest timber species planted in the orthern Jiangsu and it has important economic and ecological value. In recent years, with the increase in market demand for poplar wood and higher requirements for wood quality, the research on poplar wood property improvement is necessary. Among the wood properties, fiber length and ratio of fiber length to width directly affect the pulping performance and paper quality of poplar. In this study, we aimed to investigate the general patterns of genetic variation in fiber traits of P. deltoides and to provide the scientific basis for poplar wood improvement.【Method】Eleven ten-year-oldP. deltoides clones were used as materials. Sample trees of each clone were selected in each of the four blocks. After measuring the tree height and diameter at breast height (DBH), sample trees were cut down and the wooden discs were intercepted at 1.3 m from the ground and brought back to the laboratory for air-drying. After the samples were treated with the delignification reagent, the fiber traits were measured by optical microscope and MANUEL MORFI. The SPSS 19.0 software was used to analyze and process the data, and the genetic variation and correlation of each trait were analyzed. The K-means method was used to cluster the 11 clones, and among them, the clones with excellent fiber and growth characters were selected.【Result】The regression analysis of values measured by the two methods showed thatR² were more than 0.9, thus the MORFI measurement values were reliable. The fiber length and fiber length to width ratio increased with the increase of forest age. The average values of fiber length, fiber width, and fiber length to width ratio of 11 clones were 1 107.8 μm, 22.3 μm and 49.57, respectively. There were significant differences in fiber traits among different clones. The heritability of the three fiber traits was 0.764, 0.832 and 0.590, respectively. The correlation coefficients between fiber length and growth character were between 0.21 and 0.25, and those between fiber width and growth character were between 0.39 and 0.48. The correlation coefficients between fiber length to width ratio and growth character were between -0.07 and -0.02. Eleven clones could be classified into three categories.【Conclusion】The MANUEL MORFI is able to quickly and accurately measure fiber length and width, which was suitable for the measurement of mass data, but the results had to be adjusted by regression. The values of fiber length and ratio of fiber length to width increased with the increase of forest age, which implies that the pulping performance of wood was better with the increase of forest age. Correlation coefficients between different ages were high. These results provided a possibility for early selection of fiber traits in P. deltoides. Genetic variations in fiber traits were observed in different clones, and the heritability of each trait ranged from 0.366 to 0.832. The independent selection method could be used for genetic improvement of P. deltoides. Among all 11 clones, 2-2, 4-6, 4-45 and 4-50 had better fiber and growth properties than those of the rest, and these clones should be further tested to create conditions for new breeding.