Evidence for baroclinic tide origin of internal solitary waves (ISWs) in the northeastern South China Sea is presented, based on 116 internal wave packets observed in satellite images from 1995 to 2001. These wave packets can be divided into two types, a single-wave ISW packet containing only one ISW with/without an oscillating tail, and a multiple-wave ISW packet composed of a group of rank-ordered ISWs. All of the 22 single-wave ISW packets occur in the deep water zone. It is suggested that the ISWs, instead of being generated by the lee-wave mechanism, are developed by nonlinear steepening of the baroclinic tides, which are produced by the strong tidal currents flowing over a ridge in Luzon Strait. This suggestion is verified by an ERS-2 SAR image, which records such an evolution process from a baroclinic tide to a single ISW in its spatial domain.