An ACC deaminase gene from Sinorhizobium meliloti1021 was introduced into Sinorhizobium fredii HH103 and Sm1021 (providing extra copy of ACC deaminase gene) using the conjugation of Escherichia coli S171 and Rhizobium.These genetic engineering strains were inoculated to host plants to study the effect of ACC deaminase on symbiotic nitrogen fixation and competitive nodulation ability. The results showed that overexperssion of ACC deaminase in Sm1021 increased the nodule number and fresh weight in alfalfa. In soybean,inoculated with HH103 containing ACC deaminase also increased the nodule number,the dry weight and competitive nodulation ability. But the nodule fresh weight and nodule nitrogenase enzyme activity had no significant change both in alfalfa and soybean. 