With the increasing number of surgery or operation, perioperative infection has become one of problems that have been found more often. Surgical site infection is the most common perioperative infection causing increased hospitalization stay, high cost, morbidity and mortality rate. Infection occurs within 30 days after the operation on surgical site or within one year if implant is in place. Such infection may be prevented through several ways including some aspects of health-care provider, operating-room environment, and adequate preoperative preparation of the patients. Antibiotic prophylaxis is one of important modalities in preventing surgical site infection. Antibiotic prophylaxis administration significantly reduces the incidence of surgical site infection up to four-fold of decrease. Short-term antibiotic is given prior to incision in order to reduce the contamination of bacterial inoculums during surgery. The decision to administer antibiotic prophylaxis should be made by considering their risk and benefits. One of them includes utilization of the NNIS (National Nosocomial Infections Surveillance) score system, which considers three factors, such as wound class, ASA physical status scale, and duration of operation according to the NNIS Survey. The selection on timing and appropriately administered antibiotic prophylaxis is critical to maximize the benefits.