The optimal design of an auto-catalyst needs a good compromise between the pressure drop and flow uniformity in the monolith. One of the effective methods to achieve this goal is to use the concept of a radially variable cell density. But this method has not been examined with respect to its usefulness in terms of chemical behavior and light-off performance. In this study, a multi-dimensional performance prediction of catalyst coupled with a turbulent reacting flow simulation has been used to evaluate the benefits of this method from the simultaneous viewpoint of fluid dynamics and chemical response during warm-up. The results showed that the combined monolith with 93/73 and 93/62 was very prospective for improved light-off performance and reduced the pressure loss due to both the balanced space velocity and the efficient usage of the geometric surface area of the channels. It was also found that the air distribution between the different cell densities greatly affects not only pressure loss and flow uniformity but also the light-off pattern. In this study, thermal durability was also examined by using simple correlation governed by mechanical and thermal properties. It was found that based on the thermal shock parameter, the cell combined monolith shows weaker thermal resistance than the conventional monolith. (C) 2002 Published by Elsevier B.V.