The purpose of this study was to compare the effects of preoperative and postoperative iliohypogastric ilioinguinal nerve block (IINB) performed on 30 pediatric patients having elective unilateral herniorrhaphy for postoperative pain control and to discuss the theory of post-injury hypersensitivity of the nerves system. The patients were assigned into two groups (A = 15, B = 15) randomly. Both groups were given GA during the surgical procedure except that group A received IINB 15 minutes prior to skin incision and group B immediately after skin closure. The local anesthetic used was 0.25% bupivacaine and the dosage was 1 mg/Kg. The heart rate, BP, and anesthetic concentration (halothane) were recorded during the procedure and postoperative pain was assessed by using the modification of the Children's Hospital of Eastern Ontario Pain Scale (CHEOPS) behavior pain score which were taken at 1 hr, 3 hr (POR), 6 hr, and 24 hr (ward) postoperatively. Group A had less pain at 1 hr and 3 hr than group B postoperatively and is statistically significant. There was no statistically significant in pain score at 6 hr and 24 hr between the two groups.