By using luxAB as the report genes, 3.2 kb parCBA/DE gene fragment from pTR102 was inserted into pLAFR3 which contained a 3.7 kb enhancing fragment and deleted its cos site. Recombinant plasmids pHN155 and pHN156 were obtained. Contrasted plasmids pHN157 and pHN158 which contained cos site were also constructed. These four plasmids were transferred into Sinorhizobium fredii HN01 by tri-parental mating, and plasmid pHN155 and pHN158 were introduced into Bradyrhizobium japonicum TA11 by two-parental mating. The stability of the above plasmids was compared under free-living conditions and the results showed that the parCBA/DE could obviously enhance plasmid stability both in S. fredii and B. japonicum, and deletion of cos site showed only less effect.