This work aims at to investigate water mass transports across the connection section between the Gulf of Thailand and the South China Sea. The model grid used is the orthogonal curvilinear grid which is constructed via cubic splines solving Laplace’s equation. For the vertical grid, the sigma coordinate is introduced to deal with significant topographical variability. The data used for calculating the water mass transports consist of bottom topography and current velocities, and are calculated from the primitive equations. A numerical method used to solve the primitive equations is the finite difference method. The results show that water mass is transported from the South China Sea into the Gulf of Thailand during May to October. For the rest of the year, water mass is transported from the Gulf of Thailand into the South China Sea. In order to validate the results, a comparison is made with Wyrtki’s results on the water mass transports of Southeast Asian Waters. Under the comparison, we found our results on track.

water mass transport, Gulf of Thailand, South China Sea, orthogonal curvilinear grid, sigma coordinate.