This paper describes an improved mathematical model to study the emission conversion effectiveness of a three-way catalytic converter, which employed detailed chemical reaction mechanism. The model also accounts for adsorption/release of oxygen in the catalyst monolith under non-stoichiometric A/F conditions. A commercial CFD code FLUENT was utilized to solve the governing equations for flow and pressure drop and to simulate the transient process in a three-way catalytic converter in a multi-dimensional manner. A comparison between simulation results and experimental data for a three-way catalyst was conducted and a good agreement was observed. Based on the improved model, some geometric parameters were studied for an elliptic monolith catalyst, which are widely used in today’s converter systems because of its advantages in packaging. Simulation results show that, in general, decreasing the ellipse ratio, i.e. the ratio of the major over the minor axis length, has favorable effect on catalyst conversion efficiency. However the significance of improvement depends on the ellipse ratio itself: if the ellipse ratio is large, the effect is strong when reducing it; as the ratio is approaching to 1, the improvement becomes less and less significant. On the other hand, increasing the converter length also improves the catalyst conversion performance, so does the increase in the cross-sectional area. For a fixed converter volume: the performance of a short catalyst with large cross-sectional area is generally not as good as the one with longer length but smaller cross-section area. For a fixed void fraction of the catalyst, a catalyst with higher cell density but thinner wall thickness, because of its increased surface area, is generally more effective than the one with lower cell density but thicker wall thickness. The improved CFD model and simulation results are proven capable of providing guidelines for practical design improvements to meet the increasingly stringent emission legislations.