Abstract: This study investigates two explosively deepening cyclones over the East China Sea. The principal results are the following: Marked temperature and moisture increase in the lower-middle troposhere, unstable atmospheric stratification and strong westerly jet stream and its associated secondary circulation are important conditions favorable to explosive development of the cyclones. The strongest vertical motion, positive vorticity and upper level divergence and low level convergence were observed in explosive developmental process of the cyclones. Diabatic heating, specifically latent heat release is very crucial to its explosively deepening. The largest heating is located in northeast sector of the cyclones. Increase in the eddy kinetic energy is very marked during their explosively deepening stages. It was mainly contributed f rom the conversion of the eddy available potential energy to eddy kinetic energy. This indicates that explosive development of the cyclones is closely related to baroclinic development.