Non-steroidal anti-inflammatory agents are used to relieve pain and to reduce swelling in dental clinics. This experiment was performed to study the effect of non-steroidal anti-inflammatory agents in Sprague-Dawley rats on orthodontic tooth movement. Thirty rats were used and divided six groups of five rats each. The first group, administered saline and no orthodontic force, served as a normal group. The second group, administered saline and applied experimental force, was control group. The other four groups were administered Aspirin, Pontal, Tyrenol and Indomethacin each, and applied experimental orthodontic force by 1/4 inch elastic, inserted into the interproximal space between maxillary first and second molar in rats. All experimental rats were sacrificed after three days, and the specimens were sectioned horizontally five times serially, and counted the number of osteoclasts appeared at the compressed surface of interradicular bone on first buccal root of first molar on light microscope. The obtained results were as follows: 1. The number of osteoclast on the compressed surface of the interradicular bone on first buccal root of the first molar in the four non-steroidal anti-inflammatory agents groups decreased in contrast to control group. 2. In non-steroidal anti-inflammatory agents group, the number of osteoclast in Indomethacin group was least among the all non-steroidal anti-inflammatory agents groups. From the above results, it was believed that the non-steroidal anti-inflammatory agents may have the inhibitory effect of tooth movement during orthodontic treatment.