The purpose of this study was to observe the pattern of bone remodeling in the periodontal ligament of dog, in which experimental tooth movement was performed. A control and 5 experimental dogs, one and half year in age, were studied. Light force (50-75g) was applied by placing open-coil spring between left mandibular premolars; heavy force (250-300g), between right mandibular premolars. Experimental dogs were sacrificed at 12 hours, 1, 3, 7 and 14 days after force application, respectively. And soft X-ray films were obtained and read on the sectioned periodontal tissue around mandibular premolars. The results were as follows : 1. New bone formation began to be observed in tension side at 7-day and increased at 14-day ; No difference was observed between light force group and heavy force group. 2. Bone resorption was observed as a shape of destruction of lamina dura at 3-day and increased gradually at 7-day, 14-day.