Disclosed herein is a method of processing rounded toothbrush bristles using synthetic bristles. The method includes cutting one end of a filament bundle of synthetic bristles, mechanically polishing the cut end of the filament bundle of synthetic bristles, heat treating the polished end of the filament bundle of synthetic bristles, and caustisizing the heat-treated end of the filament bundle of synthetic bristles into a round shape. In this method, at least one end of the toothbrush bristle is softly rounded to protect gum damage due to irritation of the one end of the toothbrush bristle and to provide a gum massaging effect for healthy gum with the rounded end of the toothbrush bristle.