This study aimed to investigate the effects of dexamethasone intramuscular injection 1h preoperatively, in reducing facial swelling, pain and trismus after lower impacted third molar (LITM) surgery. Twenty healthy Thai patients with both LITM surgical extraction were enrolled in the study. The washout period was 1 month after the first operation. Clinical assessment of the facial swelling, pain and trismus were measured before and after operation for 7 days and the patient's total analgesic consumption was recorded. The level of significance used in the statistical decisions was P<0.05. Preoperative intramuscular injection of single-dose 8mg dexamethasone reduced postoperative swelling after LITM surgical extraction significantly on the second postoperative day, but immediately after surgery and on day 7 after the surgical extraction, no significant difference was found between the dexamethasone and control groups. Dexamethasone also reduced postoperative pain after LITM surgical extraction significantly on postoperative days 2 and 7. Additionally, the amount of paracetamol decreased significantly. There were no significant differences in trismus in the study and control groups 7 days after LITM operation. Single-dose intramuscular injection of dexamethasone can reduce postoperative facial swelling and pain, without affecting trismus after LITM surgical extraction.