A five-layer primitive equation model was used in the study of rapid cyclogenesis over sea. The numerical simulations show that the Tibetan Plateau, large-scale baroclinicity and heat flux from the sea surface are the three important agents to produce cyclonic vorticity along the east coast of China. The diagnostic computation of a 24 h control simulation indicates that the nonlinear vorticity advection, baroclinicity and diabatic heating are also responsible for the development of the extratropical cyclones over seas.