The shape of unit cell of catalytic converter has great influence on the conversion efficiency and pressure drop characteristics. Therefore, the properties of design parameters of catalyst monolith were analyzed and the parameters of various cell shapes of catalyst were compared.Also, the numerical study of a three dimensional compressible flow in a Close-coupled Catalyst Converter (CCC) system was performed to investigate the flow characteristics and the flow distribution of exhaust gases. Unsteady flow analysis shows that severe interferences of each pulsating exhaust gas flow as well as geometric factors (junction, mixing pipe, cell shape etc.) influence greatly on the flow uniformity and flow characteristic in substrate. The results can be applied for the catalytic converter design.