Ketjen Black EC 300J with an extremely high mesoporous area and electrical conductivity, was used as an additive in the cathode catalyst layer to improve the DMFC (direct methanol fuel cell) performance. Ketjen Black EC 300J and the catalyst ink were characterized by TEM. The cathode catalyst layers were characterized by SEM, in situ cyclic voltammetry and I–V curve measurements. Ketjen Black EC 300J additive increased the dispersion extent of Pt black particles and improved the Pt utilization. In addition, the pore size and porosity was increased when Ketjen Black EC 300J was added into the cathode catalyst layer. The cathode catalyst layer with Ketjen Black EC 300J additive showed a greater single cell performance than the cathode catalyst layer without any additive, especially in the air-breathing mode. These results suggested that the performance improvement was attributed to the increased Pt utilization, oxygen diffusion and water removal capability when Ketjen Black EC 300J was added into the cathode catalyst layer.