Objective To determine the median effective concentration(EC50)of ropivacaine at the volume of 0.5 mL/kg for ultrasound-guided interscalene brachial plexus block in children aged 6 to 10 years.Methods A prospective dose-finding trial was conducted based a sequential Dixon up-and-down allocation.We recruited children aged 6 to 10 years who were scheduled for unilateral surgery on upper extremity regions in our hospital from April to December 2022.All of them were subjected to general intravenous anaesthesia combined with ultrasound-guided interscalene brachial plexus block.The ropivacaine volume for each patient was 0.5 mL/kg.The concentration of 0.2%for the first patient,subsequent concentrations were adjusted based on the block effect of previous patient,increase or decrease by 0.02%.The trial was stopped when there were 7 turning points.Isotonic regression and Bootstrap were used to calculate the values of EC50 and 95%effective concentration(EC95),along with their 95%confidence intervals(CI).General data,incidence of adverse events,and score of postoperative pain were recorded.Results A total of 26 children aged 6 to 10 years were included.The EC50 of ultrasound-guided interscalene brachial plexus block was determined to be 0.091%(95%CI:0.077%～0.105%),and the EC95 was estimated to be 0.117%(95%CI:0.110%～0.118%).Successful blockade was achieved in 16 cases(61.5%),while 10 cases(38.5%)failed.No statistical differences existed between successful and failed cases regarding sex,age,body weight,surgical site,surgical laterality,operative time,or anesthesia duration.None of the patients experienced adverse events such as pneumothorax,vascular injury,or Horner's syndrome,and the score of postoperative pain were all<6 by modified Children's Hospital of Eastern Ontario Pain Scale.Conclusion In children aged 6 to 10 years,the EC50 of ropivacaine is 0.091%at a volume of 0.5 mL/kg for ultrasound-guided interscalene brachial plexus block,and this low dose of regional anesthetic can reduce the risks such as systemic toxicity and direct neurotoxicity.