Abstract: This study investigated the role of DnaA in the relationship between cell cycle and heterocyst differentiation using Anabaena PCC 7120 as the model organism. When providing combined nitrogen, deletion or overexpression of DnaA did not affect cyanobacterial proliferation and heterocyst differentiation. Upon deprivation of combined nitrogen, DnaA deletion (Malr2009) showed no difference in heterocyst frequency (8.57%) compared with the wild type(8.64%) at 24h, and the proliferation rate of Malr2009 was similar to the wild type, DnaA deletion did not affect cyanobacterial frequency of heterocysts or proliferation rate, but the overexpression of DnaA protein (Oalr2009) reduced the frequency of heterocysts by 20% compared with wild type, and increased the cell proliferation rate by 30% with about 1.2 at the 12^th day, higher than that of the wild type (about 0.9 at the 12^th day). In conclusion, although DnaA is not necessary for the growth of Anabaena sp. PCC 7120 cells, but under deprivation of combined nitrogen, the increased free DnaA inhibited the frequency of heterocysts.