Abstract: Five strains with high nitrogenase activity from twenty one Poplar clones and half families were isolated and selected,and then,respectively inoculated to Poplar clone NL 80106 tissue culture seedlings′ rhizosphere.After two months′ survey of seedlings′ growth characters,the result showed that the growth of tissue culture seedling inoculated with nitrogen fixing bacteria are all better than that of the control. The dry weight increased 17.9%～82.5%.The height increased 11.8%～29.6%. Nitrogen content of leaves increased 3.7%～21.5%. The increase of leaves′ phosphorus content ranged from 1.5%～57.0%. Original position surveying showed that nitrogenase activities of seedling were 12.25～45.81 nmol C 2H 4 per seedling in an hour.The inoculating effect of 3rd and 71st strains were the most remarkable. The seedling′s height,dry weight,the ground diameter and number of leave all reached remarkable level, particularly the chlorophyll,nitrogen and phosphorus content of seedling leaves inoculated with 71st strain were all the highest, its acetylene reduction nitrogenase activity was also high. This showed that they have relatively obvious associative nitrogen fixation with tissue culture seedlings of Poplar clone NL 80106, simultaneously improved phosphorus supplies condition and obviously promoted growth of tissue culture seedlings.