The effect of stb locus of E. COLI IncFII plasmid NR1 on the stability of chimeric plasmids was investigated. First, we have isolated the stability locus (stb) from E. coli NR1 plasmid and then inserted into the three different vectors, pUC8, YRp17 and YEp24. By examining their stability in E. coli and yeast, we showed that the recombinant plasmids containing stb locus were resonably stable. Also, by comparing the amounts of the rDNA fragments per haploid genome with those of the plasmid fragments, we showed they copy number of recombinant plasmids was not increased. Consequently, the stb locus of E. coli IncFII plasmid NR1 stabilized the chimeric plasmids but did not affect the replication or copy number of plasmids.