Abstract: Taking Populus ussuriensis sample trees as objects, the crosssectional and longitudinal variation of the vessels in the mature (18-48 years old) tree stems was studied from the microstructure point of view by means of quantitative anatomic methodology with timber anatomy and measurement system. The results showed that the crosssectional vessel size of P. ussuriensis timber increased along with the age increment. But there was large deviation of correlation between the vessel size indexes and the tree age, the complex correlation coefficient R2 was from 0.85 to 0.31.The longitudinal variation of the vessel size of P. ussuriensis timber was that the vessel length both of the early wood and late wood increased along with the tree height increment before the tree reached 4m tall, then decreased above 4m tall of the truck. The vessel diameter of the late wood decreased with the tree height increment.