Objective To investigate the effect of ultrasound-guided serratus plane block onefficacy of postoperative analgesia in patients undergoing radical mastectomy. Methods From January 2013 to December 2016, sixty-eight ASA physical statusⅠorⅡ patients scheduled for radical mastectomy under general anesthesia were randomly divided into two groups (n = 34 each): SP block group (observation group) and control group. After induction of anesthesia, ultrasound-guided homolateral SP block was performed, and 0.375% ropivacaine 20 ml was injected in observation group, while the equal volume of normal saline was used instead in control group. The patients received patient-controlled intravenous analgesia (PCIA) with sufentanil after operation in two groups. The scores of prince henry pain scale (PHPS) were evaluated at the 2nd, 4th, 8th, 12th and 24th hour (T1, T2, T3, T4, T5) after operation. The consumption of sufentanil, propofol and remifentanil was recorded.Meantime, the times of successfully delivered doses (D1) and attempts (D2) within 24 h after operation were added up, and D1/D2 was calculated. The adverse effects were also recorded. Results The scores of PHPS at T1 [(1.4 ± 0.5) scores vs. (3.1 ± 0.6) scores], T2 [(1.5 ± 0.6) scores vs. (2.9 ± 0.8) scores], T3 [(1.7 ± 0.5) scores vs. (2.8 ± 0.7) scores], and the consumption of sufentanil [(53.4 ± 16.7)μg vs. (87.8 ± 24.5)μg], remifentanil[(288.7 ± 42.1)μg vs. (376.5 ± 61.6)μg] were significantly lower in observation group than those in control group, while D1/D2 (0.89 ± 0.12 vs. 0.57 ± 0.18) was higher (P<0.05). The incidence of adverse reactions was significantly lower in observation group: 0 vs. 20.6% (7/34), P<0.05. Conclusions Ultrasound-guided SP block reduces the perioperative opioids consumption and enhances the efficacy of postoperative analgesia in patients undergoing radical mastectomy.