Objective To determine the optimum dose of dexmedetomidine for lumbar plexus combined with sciatic nerve block when mixed with ropivacaine.Methods Eighty ASA physical status Ⅰ or Ⅱ patients of both sexes,aged 18-63 yr,weighing 47-83 kg,scheduled for elective ankle joint surgery,were randomly divided into 4 groups (n =20 each) using a random number table:ropivacaine group (group R) and different doses of dexmedetomidine mixed with ropivacaine groups (RD1-3 groups).Lumbar plexus block was performed by using psoas-compartment approach guided by a nerve stimulator.0.5％ ropivacaine 20 ml was injected in group R.0.5％ ropivacaine 20 ml containing dexmedetomidine 1.0,1.5 and 2.0 μg/kg was injected in RD1 3 groups,respectively.Labat's sciatic nerve block was performed,and 10 ml of the corresponding drug was injected in each group.The onset time and duration of sensory and motor blockade,and side effects such as cardiovascular events and excessive sedation were recorded.Results There was no significant difference in the onset time and duration of sensory and motor blockade between the four groups.The duration of sensory and motor blockade was significantly longer in RD1-3 groups than in group R,in RD2 and RD3 groups than in group RD1,and in RD3 group than in RD2 group.The incidence of over-sedation and bradycardia was significantly higher in RD3 group than in RD1.2 groups.Conclusion The optimum dose of dexmedetomidine is 1.5 μg/kg for lumbar plexus combined with sciatic nerve block when mixed with ropivacaine.