The promotion of mycelial growth, fruit body yield and quality of Pleurotus ostreatus by Bradyrhizobium japonicum were investigated. Four areas of spread bacterial suspension of B. japonicum Hn03 were evenly distributed around the colony of P. ostreatus Td300 in PDA plate. 10 µl bacterial suspension was inoculated in each area, and its concentration was 3.6×10⁷ ml^-1. The control was bacterial suspension replaced with sterile water. Compared to the control, in the B. japonicum Hn03 and P. ostreatus Td300 co-cultivation plates, the mycelia of P. ostreatu were dense and thick, mycelial growth rate and protein content were increased by 28.17% and 22.04%, respectively. 1 ml bacterial suspension of B. japonicum HnO₃ were inoculated with P. ostreatus Td300 spawn in test tube containing 60 g cotton seed substrate, the mycelial growth rate was faster by 8.01% than the control. 5 ml bacterial suspension of B. japonicum Hn03 were inoculated with P. ostreatus Td300 spawn in each end of cultivation bag, in which 400 g cotton seed substrate was bagged. The duration of mycelia complete colonization of the substrate was significantly short, as well as biological efficiency, N, P, K, Ca, Mg, Fe, Cu, and Mn contents of fruit body were increased to 18.80%, 12.28%, 8.09%, 9.34%, 15.63%, 12.50%, 7.54%, 22.11, and 15.98% than that of control, respectively. Promotion of B. japonicum to P. ostreatus was include accelerating mycelial growth, shortening cultivation period, increasing yield and quality of fruiting body. The mechanism was associated nitrogen·fixation and enhancement the available of mineral elements.