Large amplitude internal solitary waves(ISWs) often exhibit highly nonlinear effects and may contribute significantly to mixing and energy transporting in the ocean.We observed highly nonlinear ISWs over the continental shelf of the northwestern South China Sea(19°35’N,112°E) in May 2005 during the Wenchang Internal Wave Experiment using in-situ time series data from an array of temperature and salinity sensors,and an acoustic Doppler current profiler(ADCP).We summarized the characteristics of the ISWs and compared them with those of existing internal wave theories.Particular attention has been paid to characterizing solitons in terms of the relationship between shape and amplitude-width.Comparison between theoretical prediction and observation results shows that the high nonlinearity of these waves is better represented by the second-order extended Korteweg-de Vries(KdV) theory than the first-order KdV model.These results indicate that the northwestern South China Sea(SCS) is rich in highly nonlinear ISWs that are an indispensable part of the energy budget of the internal waves in the northern South China Sea. Large amplitude internal solitary waves (ISWs) often exhibit highly nonlinear effects and may contribute significant to mixing and energy transporting in the ocean. We have observed highly nonlinear ISWs over the continental shelf of the northwestern South China Sea (19 ° 35’N, 112 ° E) in May 2005 during the Wenchang Internal Wave Experiment using in-situ time series data from an array of temperature and salinity sensors, and an acoustic Doppler current profiler (ADCP) .We summarized the characteristics of the ISWs and compression them with those of existing internal wave theories. Particular attention has been paid to characterizing solitons in terms of the relationship between shape and amplitude-width. Comparison between theoretical prediction and observation results that the high nonlinearity of these waves is better represented by the second-order extended Korteweg -de Vries (KdV) theory than the first-order KdV model. These results indicates that the northwestern South China Sea (SCS) is rich i n highly nonlinear ISWs that are an indispensable part of the energy budget of the internal waves in the northern South China Sea.