PURPOSE To evaluate the effects of immediate postoperative submucosal dexamethasone administration on postoperative pain, edema, trismus, and mandibular functions after open reduction and internal fixation (ORIF) for mandibular fractures. PATIENTS AND METHODS We conducted a prospective, randomized, controlled, double-blind study of 40 patients who required ORIF for mandibular fractures under general anesthesia. The patients were divided into 2 groups, an experimental group (n = 20) who received immediate postoperative submucosal 8 mg of dexamethasone through the surgical incision site, and a control group (n = 20) who did not receive dexamethasone. Pain was assessed using a visual analog scale (VAS) score and the frequency of analgesic consumption at the various postoperative intervals. The maximum interincisal distance and facial measurements were compared before surgery and at 24, 48, and 72 hours and 7 days after surgery. The difficulty in mandibular function after surgery was graded as mild, moderate, or severe. RESULTS The analgesic drugs required 2 hours after surgery and the VAS score 72 hours after surgery were significantly less (P < .05) in the experimental group than in the control group. The total number of diclofenac tablets required by the experimental group was less than that for the control group, but the difference was not statistically significant. The control group had significantly increased swelling (P < .05) compared with the experimental group from preoperatively to 24 hours postoperatively (experimental group 0.115 ± 0.143, control group 0.253 ± 0.173). No statistically significant difference was present in the mouth opening or difficulty in mandibular function at the different follow-up intervals between the 2 groups (P > .05). CONCLUSION The results of our study suggest that submucosal administration of dexamethasone after ORIF for mandibular fractures is effective in reducing postoperative pain and edema.