[Objective] To reveal the functions of type III secretion system (T3SS) of peanut Bradyrhizobium sp. MM6 interacting with different host plants.[Methods] We analyzed its structural characteristics of T3SS gene clusters by BLASTn in NCBI related database, and constructed the T3SS regulatory gene ttsI mutant by homologous single exchange. We also compared the differences of symbiotic phenotype between the MM6 and its ttsI mutant by nodulating experiment.[Results] The T3SS gene clusters of MM6 included 3 regions containing 10 conserved structural genes and 8 effectors. The similarity was 83% to 93% with B. diazoefficiens USDA110 corresponding genes sequence. The total nodule number and fresh root nodule weight significantly increased in the ttsI mutant nodulating peanut and wild soybean. In soybean Zhonghuang 57, the MM6 formed red effective root nodules, whereas the ttsI mutant only formed 1 empty nodule, and the plant leaves turned yellow, and the dry weight of aboveground part was significantly lower than that of the wild type strain.[Conclusion] The T3SS of MM6 played a negative role in the symbiosis system of MM6 with peanut and wild soybean, whereas it played a positive role when nodulated with the soybean Zhonghuang 57.