The substrate material of semiconductor chips is sliced by a multi-wire saw. This processing method involves use of a slurry suspended abrasive grains in base oil. The slurry enters between the wire and workpiece, and the grains in the slurry process the workpiece by scratching or rolling actions. However, the grain actions at the processing part are not clear. Therefore, this study was performed to clarify the processing mechanism of multi-wire saw from the grain actions at the bottom of the sliced groove. The grain actions at the bottom of the groove were observed by high-speed camera. The relations between the grain actions at the bottom of the groove and the processing characteristics were clarified.