Abstract： Objective To quantitatively analyze radiation-induced trismus in patients with nasopha-ryngeal carcinoma (NPC) treated by intensity modulated radiation therapy (IMRT), and evaluate tem-poromandibular joint (TMJ) damage. Methods Between February 2001 and October 2004, 211 NPC pa-tients were treated by IMRT, with a total dose of 68 Gy, 2.27 Gy per fraction within 31 -86 days (median, 43 days). The distances between two dens incisivus medialis (DDIM) were measured before and 6 months after IMRT and then annually thereafter. Results The overall survival at 1-, 3-, and 5-year were 97.1%, 90.7% and 79. 1%, respectively. The mean irradiation doses to TMJ were 6.18 -51.36 Gy. Grade 1 TMJ damage was observed in 5.2% patients, and grade 2 occurred in one patients who had received the second course radiotherapy because of local relapse . No grade 3 or 4 TMJ toxicity was observed . Conclusions IMRT can spare the TMJ from high dose irradiation and markedly reduce severe TMJ damage.