Objective To investigate effect of dexmedetomidine on the medium effective concentration(EC50)of ropivacaine for ultrasound-guided thoracic paravertebral block. Methods Sixty ASA I orⅡpatients, aged 65-79yrs, scheduled for lobectomy under VATS, requiring ultrasound-guided thoracic paravertebral block, were randomly assigned into 2 groups(n=30 each): control group(group C)and dexmedetomidine group(groupD). Paravertebral block was performed using only ropivacaine in group C. In group D the mixture of ropivacaine and 0.75μg/kg dexmedetomidine was injected for the block.The initial concentrationof ropivacaine was 0.5% and the ratio between the two successive concentrations was 1.2. If the block was effective, the next patient received alower dose of ropivacaine, or if ineffective, a higher dose was given to the next patient. The EC50 and 95% confidence interval of ropivacaine were determined using probit regression method. Results The EC50(95% confidence interval)of ropivacaine was 0.42%(0.40%-0.45%) in group C and 0.35%(0.32%-0.37%)in group D. Conclusion The application of dexmedetomidine can decrease the EC50 of ropivacaine for ultrasound-guided thoracic paravertebral block.