This study aimed at developing a novel rebamipide liquid for an effective treatment of oral mucositis. The healing effects of a variety of liquids comprising submicronized rebamipide crystals were investigated using a rat cauterization-induced oral ulcer model. Whereas 2% rebamipide liquid comprising micro-crystals did not exhibit significant curative effect, 2% rebamipide liquids comprising submicronized crystals with moderate viscosities exhibited healing effects following intra-oral administration. The 2% and 4% optimized rebamipide liquids showed significant healing effects in the rat oral ulcer model ( p <0.01). In addition, in the rat radiation-induced glossitis model, whereby the injury was caused to the tongue by exposing only around the rat’s snout to a 15 Gy of X-irradiation, the 2% optimized rebamipide liquid significantly reduced the percent area of ulcerated injury ( p <0.05). In conclusion, the submicronized rebamipide liquid with moderate viscosity following intra-oral administration showed better both healing effect in the rat oral ulcer model and preventive effect in the rat irradiation-induced glossitis model.