Abstract: The structure characteristics of biomass and spatial distribution of root system of Pinus massoniana with different dominance were systematically investigated in the acid rain region of Chongqing, China. The results showed that the aboveground biomass accounts for more than 90%of the total biomass and the ratio of roots was less then 10%. The order of biomass ratio of different organs was trunk, branch,root, bark, needle. The depth and width of root distribution varied obviously with the dominance of individual trees. For dominant trees, the vertical root could reach a depth of 130 cm, and the horizontal range of root system with a radius of 3 m. On the other hand, it is less then 80 cm in depth and 1.5 m in radius for the non-dominant trees. The biomass proportion of roots within different diameter range was ordered as stump,large root, middle root, small root, fine root. The biomass sum of stump and large root accounts for more than 90%of the total amount of root system, while the proportion of all other root diameter classes accounts for only less than 10%. The study of fine root distribution of Pinus massoniana using soil coring method showed that the fine root was plentiful in the soil layer of 0--40 cm. It was especially concentrated in the upper soil layer (0--10 cm). Fine root biomass decreased with distance from trunk of sampled trees increased. It reached a minimum at the position of 2.0 m away from the trunk.