Background: Lower impacted third molar surgery usually involves trauma in highly vascularized loose connective tissue area,leading to expected inflammatory sequelae including postoperative pain,swelling and general oral dysfunction during the immediate post-operative phase.This study aimed to the efficacy of 8mg dexamethasone single dose injection preoperatively on postoperative pain in lower third molar surgery.Methods: A controlled randomized,split-mouth,prospectivestudy involving lower third molar surgery in 31 patients performed.The randomized sampling groups were the single dose dexamethasone 8mg(2ml)injected through pterygomandibular space,preoperatively after the local anesthesia,and 2ml of normal saline injected as a placebo and no dexamethasone.Results: Pain VAS score was significantly different in day operation and first operative day; P-value 0.00 and 0.01 respectively but not significant in third postoperative day and seventh postoperative day between control and study groups.There was significant reduction in swelling on second postoperative day and difference between second postoperative day and baseline in study group compared with control group.(P<0.05)Trismus was highly significant different on the second postoperative day and difference between baseline and second postoperative day between control and study groups; P-value 0.04 and 0.02 respectively.Descriptive statistics and independent-samples t-test to assess the significance of differences were used.Conclusion: Injection of 8mg dexamethasone into the pterygomandibular space was effectively reduced the postoperative pain and another postoperative sequalae.