To determine the relationship between sigC(all1692) and heterocyst development in Anabaena sp．PCC 7120,sigC mutant named as MsigC was constructed by double homologous recombination.Results showed that when combined nitrogen sources became limited in the growth medium，MsigC had a pronounced reduction in heterocyst frequency compared to the wild type,while heterocyst frequency was recovered by complementation with the ectopically expressed sigC gene in the complement strain CsigC.It is indicated that sigC is involved in regulating the differentiation of heterocyst.