According to previous study (6), Magnetic resonance imaging (MRI) could provide advantage findings in masticator structures in patients with trismus developing after radiotherapy for nasopharyngeal carcinoma (NPC). The abnormalities comprising radiotherapy-induced masticator muscle fibrosis or inflammation, denervation atrophy of the masticator muscles secondary to mandibular nerve damage, osteoradionecrosis change of mandibular rami, perimasticator fibrosis extending into the masticator space and post RT damage of the parotid gland. The presence of several MRI abnormalities in the masticator structures of patients with trismus after radiotherapy suggests that trismus is multifactorial. However, there was no good correlation data between the severities of trismus with these image findings and thus these changes may possible due to non-specific inflammatory and fibrosis reactions after radiation. Also, there was no correlation data between radiation dosages with these abnormality, so all these findings may also result from other causes such as surgery or chemotherapy. Therefore, we try to evaluate MRI examinations performed in patients before and after trismus establishing following radiotherapy for OCC patients, in order to identify the range and frequency of underlying diseases that may cause this complication. We retrospect the OCC patients using MRI signal abnormality scores (SA score), separating the patient according to their clinical trismus grading into three groups, the mean SA score of grade 0 patient is 8.5, grade 1 is 6.76 and grade 2 is 4.87. Compared with group 2 patients, there are statistically significant differences between group 1 (p= .04) as well as group 0 (p= .02), respectively. It means the SA score could correspond to the trismus grade (Fig. 2).