The effect of valve events on engine performance was studied using an engine simulation program. To improve the prediction accuracy, the original quasi-steady thermodynamic model developed by Poulos and Heywood was modified so that the gas flow in the intake manifold could be modelled as an one-dimensional unsteady compressible flow. Various empirical parameters in the model were calibrated against the exsiting experimental data so that the simulation bears any meaningful results.It has been known that the engine performance is more sensitive to the intake valve events than the exhaust valve events, thus only the effect of varing intake valve events was considered for the present study. The intake valve events which were varied include intake valve closing time, intake valve opening time, maximum intake valve lift time, intake valve lift, and the combination of the above. Among them, changing intake valve closing time is observed to be the most effective way of increasing volumetric efficiency, and the most promising and simplest mechanism for variable valve timing is changing both lift and timing simultaneously while maintaining the maximum valve lift timing fixed.