||The South China Sea is the largest marginal sea in the Western Pacific. Oceanic circulation in the SCS is one of the most important factors which influences the chemical and biological environment in southern Asia. In this study, a three dimensional primitive equation ocean model has been applied to developing a modeling system for the SCS to simulate circulation and to investigate the corresponding forcing mechanism. The high resolution SCS model (10 km) and Pacific Ocean model (40 km) are nested together in the system. The solutions from the Pacific Ocean model provide time-dependent, three-dimensional high-resolution lateral fluxes for the SCS model. High spatial and temporal resolution atmospheric forcing fields are adopted in the model. My investigation of the SCS circulation was divided into two parts. The first part was the model runs with a seasonal mean forcing field which aims to perform a process-oriented study to understand characteristics and forcing mechanisms of ocean currents in response to the controls of the monsoonal wind stress, the Kuroshio intrusion and other local dynamic forcing mechanisms in the SCS. The results showed that the general circulation in the SCS is cyclonic in winter and anticyclonic in summer with eddies spreading into the SCS. The Kuroshio intrudes into the SCS in winter intensifying cyclonic circulation, whereas it passes through Luzon Strait without intrusion in summer. The northeastward coastal jet is formed off the Vietnam coast and overshoots into the interior of the SCS in summer. The sensitivity study suggested that the Kuroshio and its influences on the SCS are controlled by the lateral flux from the Pacific Ocean and the wind effect is relatively minor. In the second part, the model was forced with time-dependent atmospheric and lateral fluxes to conduct a direct simulation. Realistic seasonal variability of circulation and thermal fields were obtained. The model results agree well with the observations, in particular, the model simulated well the general circulation, the evolution of the Kuroshio, the current along the Vietnam coast, as well as multiple-scale eddies in the SCS.