A fluid dynamic mathematical model of the room airflow in monolith was established by an equivalent continuum approach. The commercial CFD code STAR-CD was used to simulate multi-dimensional steady flows in automotive catalytic converters. In order to verify the fluid dynamic model of the converter, a three-hole Pitot tube was adopted to measure the velocity distribution at the rear of the monolith. Computing results are in good agreement with experiments, which means the established model is feasible and can be applied to predict the flow performances of various catalytic converters. Then, the enhanced diffusion header (EDH) converters and oblique diffuser converters with different configurations were designed and simulated by the CFD code. Simulation results indicate that EDH and an oblique diffuser can improve the flow uniformity and decrease pressure loss in the converters. These results provide a useful guide for the optimum design of automotive catalytic converters.