Tungsten carbide which is a hard and brittle material was ground by cast-iron bonded diamond wheel with ELID (Electrolytic In-Process Dressing) technique, for the purpose of getting high efficiency, super-precision machining. Three kinds of cast-iron bonded diamond wheels with different grain size were adopted to get different grinding efficiency and surface quality of workpieces. The grinding properties of cast-iron bonded grinding wheels with different grain size and the ground surface quality of tungsten carbide are discussed in this paper. The experiment results indicate that, under the same feeding amount, the grinding efficiency of the wheel with bigger grain size is higher, and it could make the dimension accuracy of the workpiece controllable, but the wheel with smaller grain size could get better ground surface quality. The two grinding phases are decided by the ratio between the size of abrasive grain and the thickness of the oxide layer on the grinding wheel.