[Abstract] Objective：To determine the median effective dilution concentration(EDC50） of 1.5ml ropivacaine for ultrasound guided median nerve block by using sequential method . Methods: Children( ASA I～Ⅱ) aged 1～3years who were scheduled for unilateral thumb sheath release surgery in Children's Hospital of Chongqing Medical University were enrolled. All children applied intravenous anesthesia combined with median nerve block. After anesthesia Induction, the ultrasound guided median nerve block was applied.After localization, the total volume of ropivacaine was 1.5ml and the test initial concentration was 0.2% according to the modified Dixon’s up-and-down method. Based on the outcome of the preceding median nerve block, a block of success or failure resulted in a 0.02% reduction or increase in concentration, respectively. Record the child's gender, age, anesthesia time (from induction until the child leaves PACU), whether remedial medication is used during the surgery, whether there are any complications (such as bradycardia, hypotension, nausea, vomiting, itching, respiratory suppression, or tracheal spasm), and follow up at 2, 4, 6, and 12 hours after the surgery to record the child's pain situation. The EDC50，EDC95 and each 95% confidence interval（CI） of ropivacaine in ultrasound guided median nerve block was calculated with the centered isotonic regression method. Results: 22 children who were scheduled for unilateral thumb sheath release surgery were enrolled. The EDC50 (95% CI) was 0.11%(0.09% ～0.15%)，the EDC95 (95% CI) was 0.18%(0.17%～0.26%), and no adverse events occurred. Conclusions: A single dose of 1.5ml ropivacaine is an effective medication for ultrasound guided median nerve block in children who were undergoing unilateral thumb sheath release surgery. The EDC50 (95% CI) was 0.11%(0.09% ～0.15%) and the EDC95 (95% CI) was 0.18%(0.17%～0.26%) in children aged 1～3 years.