Swirl ratio in the cylinder of a diesel engine is an important parameter for air/fuel mixing and combustion process. The swirl intensity generated by an intake port is measured on a steady flow rig. The swirl ratio at the end of intake process in the engine is then estimated from the steady flow test results by equations which have already been established by Ricardo and AVL. However, the existing equations are deduced from a series of assumptions. Three of them affect swirl ratio estimation significantly: a) volumetric efficiency of an engine is 100%; b) the pressure drop through the intake ports is constant during the intake process in engine operation; c) no burned gas residual is trapped in the cylinder. An accurate estimation of swirl ratio is essential during the engine combustion system development. In this paper, the authors proposed a new estimation equation of swirl ratio, in which the volumetric efficiency, the compression ratio and the pressure drop through the intake ports are three additional variables. A few swirl ratio estimations are then given using the new proposed method for different types of diesel engines. The results clearly show effects of the compression ratio, the volumetric efficiency and the pressure drop through the intake port on the swirl ratio.