Ｏｂｊｅｃｔｉｖｅ　To evaluate the efficacy of ultrasound-guided transverses abdominis plane(TAP) block for preemptive analgesia in patients undergoing caesarean section.　Methods　Ninety ASA physical status Ⅰ,Ⅱ patients, aged 20-39 y, weighed 50-80 kg, scheduled for cesarean section, were randomly divided into 3 groups(n=30) using a random number table: control group(group Ⅰ), preoperative TAP block group(group Ⅱ) and postoperative TAP block group(group Ⅲ). Bilateral transverses abdominis plane block was performed under the guidance of ultrasound immediately before induction of anesthesia and at the end of surgery in Ⅱ and Ⅲ groups, respectively. The VAS in each at 2, 4, 12, 24, 48 h after surgery with rest, the cumulative consumption of sufentanil within 24 h, the rate of rescue analgesia and times of pressing patient-controlled intravenous analgesia, the adverse reactions occurred after surgery were detected. 　Results　The VAS scores of the patients in group Ⅱ at 2, 4, 12 h after surgery[(2.3±0.4), (2.4±0.4), (2.2±0.4)] were significantly lower when compared with group Ⅲ[(3.2±0.8), (3.4±0.3), (3.1±0.5)](P<0.05). The cumulative consumption of sufentanil within 24 h, the rate of rescue analgesia and times of pressing patient-controlled intravenous analgesia of group Ⅱ[(40±5) μg, 10%, (5.8±1.4)] were significantly decreased when compared with group Ⅲ[(53±7) μg, 20%, (10.3±2.6)](P<0.05). The rate of nausea and vomiting in group Ⅱ within 24 h was significantly reduced when compared with group Ⅲ(P<0.05).　Conclusions　Preoperative transverses abdominis plane block can provide good preemptive analgesia in the patients undergoing cesarean section．