The substrate surface roughening and cleaning are very important process for pre-treatment of a substrate for thermal spraying. There are some methods for the pre-treatment. Especially, the grit blasting process is the most general process. However, some grits embed into the blasted substrate, and the embedded grits remain on it. The girt embedment is affected by various conditions of the blasting process. It is important to make known the effect of the blasting time on the residual grit amount. A carbon steel substrate was blasted by white alumina grit. The blasting time was changed. The surface roughness of the blasted substrate increased monotonically with increasing blasting time. During very short time at the starting of blasting, the residual grit amount increased with increasing blasting time. Then the amount of the residual grit decreased with increasing blasting time. The penetration depth of the residual grit on the blasted substrate denoted the same tendency of the residual grit amount. There may be a critical time that the residual girt amount becomes the local minimum value.