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The washcoat (W/C) on a catalytic diesel particulate filter (DPF) greatly alters the pore structure and wall surface condition of the original substrate of DPF, which then affects the filtration efficiency and pressure drop behavior. In the present study, we examined this W/C effect on the initial PM filtration efficiency and pressure loss by changing amounts of washcoat on a SiC-DPF. We measured particle number concentration and particle size distribution in the diesel exhaust gas downstream of the DPF by EEPS. High filtration efficiency was achieved quickly when the W/C amount was increased. We introduced new parameters, T90 and T99, which were the filtration efficiencies that reach more than 90% and 99%, respectively, of the initial DPF usage. The PM trapping mechanisms could be classified according to the PM size. Trapping of PM whose diameter is smaller than 30 nm is little affected by the W/C.