The current system in the Strait of Singapore is fairly complicated and variable because it is under the influence of major currents driven by trade winds as well as the Asian monsoons. In addition to tidal forcing, circulation in the Strait is governed by a strong hydrodynamic pressure gradient which reverses direction semi-annually and coincides with seasonal monsoon changes. In this investigation, we examine the mechanism that controls the circulation in the Strait of Singapore by using a three-dimensional circulation model forced by hydrodynamic pressure gradient, tides and winds. Model results delineate in detail the characteristics of circulation and hydrodynamic structure in the strait. These results compare favourably with the available field measurements and agree, in general, with observations. It is shown that: (1) the low frequency net transport across the strait correlates well with the seasonal hydrodynamic pressure gradient, and (2) the tidal variation across the strait have a significant effect on the circulation in the Strait.