Abstract: In this paper, a numerical simulation of land and sea breezes in southern China has been made by use of an one-level, sigma-coordinate mesoscale model developed by Mass and Dempsey.It is revealed that there are six mesoscale land and sea breezes systems in southern China according to the distribution and character istics of diurnal variation of simulating surface wind and pressure. Meanwhile, there are three mesoscale sea-breeze convergence fields corresponding to three heavy rain centers in Guangdong area.Finaly, the conversion time of land and sea breeze and its dynamic mechanics are discussed by using amplitude and phase of diurnal wave calculated from model outputs of surface.