The influence of hydrocarbon molecular size on the HC-SCR activity was investigated over Cu-MFI zeolites having different crystal sizes. In the case of n-hexane as a reductant, the catalytic activity did not depend on the zeolite crystal size. In the case of 2,2-dimethylbutane, however, the observed reaction rate depended on the zeolite crystal size, indicating that the reaction was controlled by intracrystalline diffusivity. It was verified that the catalytic activity for the SCR of NO by a larger hydrocarbon over Cu-MFI zeolite was restricted by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size.