The effect of macropore formation in catalyst supports on the oxidation activity for diesel fuel mist was examined with the goal of improving catalyst efficiency. To investigate the effect on a laboratory scale using granular samples, we designed a reaction system that supplied fuel mist to the catalyst bed directly by an air atomizing nozzle. The catalytic activity was found to be improved by macropore formation in the support. This effect was also demonstrated for a washcoated catalyst on a honeycomb with diesel engine exhaust. These results suggest that the macropore diffusion channels are secure even if fuel mist attaches to the support surface, and the reactant gas can diffuse to the interior of the support and reach the active sites. Macropore formation prevented the blockage of pores of the support surface and thus the active sites of the catalyst.