Ｏｂｊｅｃｔｉｖｅ　To evaluate the postoperative analgesic effect of ultrasound-guided pectoral nerves(Pecs) block in patients undergoing modified radical mastectomy.　Methods　Sixty adult female patients, ASA gradeⅠorⅡ, scheduled for elective unilateral modified radical mastectomy under general anesthesia were randomly allocated to receive either general anesthesia (combination of sufentanil, remifentanil and propofol) plus Pecs block(group P, n=30) or general anesthesia only(group C, n=30). Patients in group P received ultrasound guided Pecs block with infiltration of 10 ml and 20 ml of 0.375% ropivacaine into fascial planes between pectoralis major and minor muscles, and between pectoralis minor and serratus anterior muscles, respectively before general anesthesia. Patient-controlled intravenous analgesia(PCIA) with sufentanil was used for postoperative pain management. Intraoperative consumption of sufentanil, remifentanil and propofol were recorded. Sufentanil consumption, the effective (D1) and total (D2) pressing times of PCIA within 24 h after surgery were recorded, and D1/D2 ratio was calculated. The VAS scores and Ramsay sedation score at rest and movement as well as other adverse reactions such as the incidence of nausea and vomiting 2, 4, 8, 12, 24 h after operations were recorded.　Results　Compared with patients in the group C, patients in group P showed lower VAS scores (P<0.05) and similar Ramsay score 2, 4, 8, 12 h after operation. Consumption of intraoperative remifentanil and postoperative sufentanil within 24 h in group P was significantly less than those in group C, and the ratio of D1 and D2 was significantly increased in group P compared with group C (P<0.05). The incidence of adverse events such as nausea and vomiting in group P was less than that in group C(P<0.05).　Conclusions　The Pecs block is a simple feasible technique to produce satisfying postoperative analgesia for radical breast surgery.