Objective:To compare the perioperative analgesic efficacy of ultrasound-guided serratus plane block (SPB) with pectoral nerves Ⅱ (Pecs Ⅱ) block in patients undergoing modified radical mastectomy for breast cancer under general anesthesia.Methods:Sixty female patients, aged 20-60 yr, of American Society of Anesthesiologists physical status Ⅰ or Ⅱ, scheduled for modified radical mastectomy for breast cancer, were divided into 2 groups ( n=30 each) using a random number table method: SPB group (group S) and Pecs Ⅱ block group (group P). Both groups received ultrasound-guided nerve block with 0.5% ropivacaine 20 ml before induction of general anesthesia.The patients in both groups received patient-controlled intravenous analgesia.Tramadol 100 mg was intramuscularly injected as rescue analgesic.The block status of each segmental dermatome, comsumption of intraoperative remifentanil and analgesics (sufentanil in patient-controlled intravenous analgesia and rescue analgesics) within 24 h after operation, duration of nerve block, Horner syndrome, and complications such as respiratory depression, nausea and vomiting within 24 h after operation were also recorded. Results:Compared with group S, the block rate of T 5-T 7 dermatome and consumption of sufentanil after surgery were significantly decreased ( P<0.01), and no significant change was found in the consumption of remifentanil and duration of nerve block in group P ( P>0.05). No rescue analgesic was used and no nerve block-related complications and postoperative complications were found in either group. Conclusions:The ultrasound-guided Pecs Ⅱ block provides better efficacy than SPB in the patients undergoing modified radical mastectomy for breast cancer under general anesthesia.