Objective To determine the median effective concentration (EC50) of ropivacaine for ultrasound-guided brachial plexus block.Methods Fifty ASA Ⅰ or Ⅱ patients of both sexes, aged 19-72 yr, weighing 45-83 kg, scheduled for upper extremity surgery under brachial plexus block guided by ultrasound, were enrolled in this study. Brachial plexus block was performed under the guidance of ultrasound. After successful location, ropivacaine 30 ml was injected. EC50 of ropivacaine was determined by up-and-down sequential method. The initial concentration was 0.50% . Each time the concentration increased/decreased by 0.05% . EC50 of ropivacaine required for ultrasound-guided brachial plexus block and 95% confidence interval were calculated using Probit analysis.Results The EC50 of ropivacaine resulting in complete block of the brachial plexus nerve was 0.436%(95% confidence interval 0.393%-0.477% ). Conclusion The EC50 of ropivacaine is 0.436% for ultrasoundguided brachial plexus block.