OBJECTIVE To investigate the reasonable proposal of prophylactic antibiotics use in selective colorectal operation. METHODS One hundred and sixty-five patients underwent colorectal surgery were randomized to Treatment 1 (55 cases), Treatment 2 (50 cases) and Control (60 cases) group. The Treatment 1 group was given oral MgSO4 solution at the night before operation, and Cefradine 2.0 g (I.V.) during the induction of anesthesia, continued with tow times of intravenous Cefradine 2.0 g and 0.5% Metronidazole 100 ml at an interval of 12 hours in 24 hours after the operation. The Treatment 2 group was given the same treatment as Treatment 1, but the antibiotics would not be withdrawn until 3-5 d after operation. On the basis of the treatment of Treatment 2 group, the Control group was given oral antibiotics 2-3 days before operation. Postoperative complications including surgical site infection, stoma leakage, dysbacteriosis, and WBC, body temperature, days of hospitalization and antibiotic expenses in the three groups were observed and compared. RESULTS There was no significant differences in surgical site infection, stoma leakage, WBC counting and its change, body temperature and hospital stay among the three groups (P > 0.05). The incidence rate of dysbacteriosis in Control group was significantly higher than that in Treatment 1 group (P < 0.05). The antibiotic expenses in the Treatment 1 group was significantly lower than those of the other two groups (P < 0.05). CONCLUSIONS Prophylactic antibiotic use during the induction of anesthesia and 24 hours after operation was reasonable in selective colorectal operation, it can prevent the surgical site infection effectively with good social-economic effects and fewer side effects.