UV light irradiation to a photocrosslinkable chitosan (Az-CH-LA) resulted in an insoluble and flexible hydrogel within 30 s. The purpose of this study was to evaluate the ability of the photocrosslinkable chitosan to inhibit bone formation in the bone defects. A 5-mm-diameter defect was made in the rat calvarium, and then photocrosslinkable chitosan was implanted and irradiated with UV for 30 s. Furthermore, a 2-mm defect was made in the fibula of a rat hind leg, and then photocrosslinkable chitosan was implanted and irradiated with UV. Bone formations in the rat skull and fibula defects with photocrosslinkable chitosan hydrogel were significantly prevented for 8 weeks. Thus, the chitosan hydrogel has an inhibitory effect on bone formation.