Objective To evaluate the efficacy of ultrasound-guided sacral block in the obese patients by comparing it with the traditional positioning method.Methods Sixty obese patients,aged 25-50 yr,weighing 56-80 kg,with body mass index ≥ 30 kg/m2,scheduled for elective anorectal surgery,were randomly divided into 2 groups (n =30 each) using a random number table:ultrasound-guided sacral block group (group U) and traditional positioning method group (group T).The sacral canal puncture point was located via either ultrasonic or traditional positioning method.After successful puncture,1％ lidocaine 20 ml was injected.The number of patients in whom puncture was successful at first attempt and the number of patients in whom the time for puncture ≤ 1 min were recorded.The adverse reactions were observed.The efficacy of sacral block was evaluated.Results Compared with group T,the success rate of puncture at first attempt was significantly increased (P ＜ 0.05),and no significant changes were found in the number of patients in whom the time for puncture ≤ 1 min or efffcacy of sacral block in group U (P ＞ 0.05).No adverse reactions were found in the two groups.Conclusion Ultrasound-guided sacral block can raise the success rate of puncture at first attempt and provide similar efficacy of block in the obese patients when compared with the traditional positioning method,and thus has significant clinical value.