Surface roughness in rough machining affects the quality and processing capacity of finishing process. The smaller grain size obtains the smoother surface. In this paper, the recently developed brown AI2O3 grinding wheels in Vietnam, which can machine up to speed of 45m/s, has been observed. There are four grinding wheels with different of grain sizes #36, 46, 60 and 80 were used to machine hardened C45 steel in order to evaluate grindabality of grain sizes. The results showed that smoother surface could be obtained in shorter time by coarse grains, e.g., the surface roughness of 0.34 om could be obtained by #36 grinding wheel in 14 grinding times, while grinding time of #46, 60 and 80 were 15, 19 and 27 grinding times, respectively. On the other hand, the bigger grain size, i.e. #36, shows advantage in rough machining to obtain smoother surface than that of fine grains.