Abstract:169 clones chosen from 30 localities of Populus cathayana Rehd. covering ten provinces were colleced for analysis of DNA polymorphisms between populations and within population by RAID. The result showed that the degree of DNA polymorphism between populations was much larger than that within population, and the degree was greater between eastern and western population than that between the neighbor populations. The UPGMA and Neighbor-Joining cluster analysis based on net genetic distance were used to investigate phylogenetic relationship among populations. They beth showed apparent genetic differentiation between eastern and western population. The dendrogram had correspondence with geographic distribution region of P. cathayana. The western revealed much more polymorphisms than the eastern population. Therefore, the western populations had more important role in the origin and evolution of P. cathayana. According to the relationship between diversity center and origin centerof species, it is possible that P. cathayana originated from the northwestem China and then spreaded to the eastern region forming modem distribution region.