In this report, the velocity vector field around the rotating straight grinding wheel were visualized by a smoke-wire method and an image analysis such as a gradient method developed by Miike et al. Consequently, we found that the each method for visualization were avilable to estimate the velocity vectors quantitatively. We also found that the air flow behavior around rotating straight grinding wheel was suggested complicated flow pattern such as the suction and injection which were caused by the internal flow of the porous body. The effect of thermal convection on the heat transfer should be considered in the calculation of heat balance in grinding.