Abstract: Both the primary forcing mechanisms and dynamical/thermdynamical distributions of two explosive cyclones over the Northwestern Pacific Ocean were investigated using synoptic meteorology, potential vorticity dynamics and Lagrangian generalized Z-O equation diagnosis. The results demonstrated that thermodynamical forcing dominated the explosive development of marine cyclones, i.e. explosive development occurred as Laplacian temperature advection reflecting the baroclinicity of atmosphere. Laplacian cumulus heating and turbulent heating combined to force significant geostrophic vorticity increases while more contributions resulted from Laplacian cumulus heating. Dynamical forcing of vorticity convection played relatively small role while explosive development progressed. The primary thermodynamical forcing processes that acted to inhibit development were the sensible heat transfer from air to sea and the adiabatic cooling whereas the dynamic forcing process that exerted a consistently strong damp influence was the friction dissipation. Morover. the three primary forcing processes also resulted in the much slower development or decay of cyclones. The initiation factors that triggered the explosive development of cyclones differed between the two cases. Either the vorticity-convection or both temperature convection and large-scale condensation heating can drive to initiate the explosive development of cyclones. The examinations of integrated-averaged distribution and vertical structure of dynamical/thermody-namical forcing terms revealed further some important dynamical/thermodynamical distribution characteristics.