It has been reported in our previous report that electrically conductive CVD diamond (hereafter, EC-CVD diamond) having specific resistance of the order of 1x10-3 Ω⋅m shows almost zero electrode wear even at the short pulse duration of 3μs on a die-sinking EDM machine. In this research EDM properties are investigated in the context of the pulse on time in oil and water. It is found that the electrode polarity has an effect on the electrode wear of the EC-CVD diamond electrode in oil. In the test in oil, EC-CVD diamond shows zero wear at positive [+] electrode polarity but a higher wear at negative [-] electrode polarity than that of the copper electrode. Furthermore, the EC-CVD diamond electrode showed very less wear than copper even in water. These results in oil and water may be due to the high thermal diffusivity, an increased rate of carbon adhesion and the high thermal stability of the EC-CVD diamond electrode. Workpiece surface roughness using the EC-CVD diamond electrode is about 20~50% lower than using the copper electrode under the same EDM conditions. From the single pulse discharge experiment with EC-CVD diamond and copper electrodes, the improvement in roughness is explained by the dispersion of the discharge in the case of EC-CVD diamond due to its rather high specific resistance.