Physical mechanisms which determine the seasonal trend of the northwestern Pacific level are considered. The contribution of steric, tidal, and static effects of the atmospheric pressure and the radiation budget are estimated. Based on analysis of the altimetric satellite TOPEX/POSEIDON information, it is demonstrated that besides the above-said effects the seasonal trend of the northwestern Pacific is also related to the dynamic processes, especially low-frequency gradient-vortex waves. The relative contribution of gradient-vortex, barotropic, and baroclinic waves to the annual and semiannual sea level oscillations is studied and the parameters of these waves are determined.