One strain SC18-cfp marked with cfp gene was obtained by thri-patental mating.The results showed that cfp gene could express in SC18-cfp and kept stabile,by detecting the expression and stability of SC18cfp.The result of pot experiment in nitrogen-free sand indicated that the competitiveness and effectiveness of the marked strain were not significantly different with the parental strain.Using the cfp marked gene to study competitive ability of SC18-cfp with soil indigenous rhizobia under controlled conditions in field experiment on three phaseolus culitivars,we found that the SC18cfp had higher nitrogen fixing efficiency and stronger competitive ability than that of the soil indigenous strains.Its nodule occupancy on three cultivars of phaseolus vulgaris was 57.8%,60.0%,55.6% respectively.Inoculation of strain SC18 could significantly increase the plant shoot dry weight,chlorophyl content,total nitrogen and the plant yield.