Abstract: Root is an important function organs of the plants，has an irreplaceable role in material cycle and ecergy flow of the forest ecosystems，and root biomass is one of important data to accurately evaluate carbon storage and carbon function of forest ecosystem. This paper adopt the methods of harvest standard tree to research root biomass of three main forest tree species in Xinjiang，And vertical distribution of below ground root biomass of three species and its variation among different age structure were also investigated in order to understand the ecosystem function in the Xinjiang forest ecosystem. The aim was to provide scientific reference for the study of forest ecosystem root biomass and productivity，and the basic data for the further estimation of cabon storage and carbon sink of the forest in Xinjiang.  Results showed that：（1）Root biomass of Larix sibirica Ledeb，Abies sibirica Ledeb and Populus alba var pyramidalis Bge are 43.409 t/hm²，42.152 t/hm² and 16.19 t/hm². （2）Root biomass declines with increasing soil depths. Approximately more than 88.37% root biomass distributes in 0～40 cm soil layers and approximately 8.3% root biomass distributes below 40 cm depth. （3）With increasing of stand age，the root biomass always increases. of them，the increasing rate of root biomass of middle-age achieves maximum. The biomass proportion of roots with different diameter ranged as stump，middle root and fine root decreases with age increase. But the root stake and capillary root biomass are different，the proportion increases with age increase. （4）In top soil layers，the proportion of root biomass of young and middle age is in a higher level. In deeper soil layers，root biomasses of three young trees are all in a lower level.（5）Root productivity of Populus alba var pyramidalis Bge，Larix sibirica Ledeb and Abies sibirica Ledeb is 1.0708 t/ （hm²·a），1.0144 t/ （hm²·a） and 0.9107 t/ （hm²·a），respectively.

Key words: forest stand, the roots, biomass, age, diameter class