HC and CO emissions during the cold start contribute the majority of the total emissions in the legislated driving cycles. Therefore, in order to minimize the cold-start emissions, the fast light-off techniques have been developed and presented in the literature. One of the most encouraging strategies for reducing start-up emissions is to place the warm-up catalyst, in addition to the main under-body catalyst, near the engine exhaust manifold. This study numerically considers three-dimensional, unsteady compressible reacting flow in the warm-up and main catalysts to examine the impact of a warm-up catalyst on thermal response of the main catalyst and tail pipe emission. The effects of flow distribution and loading condition on the temperature distribution and emission performance have also been investigated. The present results show that flow distribution has a great influence on the temperature distribution in the monolith at the early stage of warm-up process and optimal catalyst distribution of high loading at the entrance has no effect on conversion improvement when the space velocity is too fast for reaction to complete within high loading region (above SV=100,000 hr-1 ).