Herein, the nanosized MOR zeolites were applied for selective catalytic reduction of NOx with NH3, and the effect of particle size on catalytic performance was studied in detail. Accordingly, three kinds of Cu-MOR with different particle sizes (0.23, 0.96, and 2.20 mu m) were prepared and used for removing NOx from diesel exhaust. Our results demonstrated that Cu-MOR-0.23 with the minimal particle size had the best catalytic performance. Decreasing the particle size can effectively shorten the length of the C axis (12-MR channel), which is beneficial for the diffusion of substrates and products. This advantage was also verified by the Weisz-Prater criterion that the Cu-MOR with the smallest particle size has the least hindered diffusion of the mass transportation. Our work provides a new approach for promoting the catalytic performance of conventional cheaper catalysts used in the removal of NOx from diesel exhaust.