In this layered three dimensional numerical ocean model the seasurface is free to allow simultaneous simulations of surface and internal tides. Since the propagating speed of the external wave is much greater than those of internal waves, the Courant-friedrichs-lewy (CFL) condition for external mode is much more restrictive than that for internal modes. In the present model an alternating direction implicit (ADI) scheme is employed for external mode computation to avoid the restriction of CFL condition. The internal modes are computed with an explicit treatment. The model has been preliminarily applied for simulation of internal tides generated by surface tides in the northwestern South China Sea.