Ｏｂｊｅｃｔｉｖｅ　To compare the sedative effects and safety of dexmedetomidine（Dex） and midazolam by continuing infusion during microsurgery under regional block.　Methods　Sixty forearm or hand emergency wounded patients scheduled for microsurgical operation under regional anesthesia were randomly assigned to Dex group （group D） and midazolam group （group C）. Observers assessment of alertness/sedation scale （OAA/S） and bispectral index （BIS） values were used to observe the sedative effect of Dex and midazolam during surgery. In all patients， the sedative drugs were intravenously administrated after the completion of regional block anesthesia. Moreover， BIS， mean arterial pressure （MAP）， heart rate （HR）， oxygen saturation and OAA/S scales were recorded at time-points of before administration （T0）， beginning of surgery （T1）， 15（T2）， 30（T3）， 60 min （T4）， post-operation instant（T5） and 1 h post-operation （T6）. The side effects of patients of the two groups were recorded. The cases of forgetting intraoperative events and the incidence of postoperative vascular crisis were also recorded.　Results　After administration of Dex， MAP and HR in group D at times of T1-T5 were significantly lower than T0（P<0.05）. HR at time-points of T1-T5 in group C were no significantly varied （P>0.05）. MAP at time-point of T1 in group C was significantly decreased than T0（P<0.05） and MAP at time-points of T2-T5 in group C were not significantly varied （P>0.05）. Comparison between groups， HR at times of T1-T4 in group D was significantly lower than that in group C. The values of BIS and OAA/S at time-points of T1-T5 were all significantly lower than T0 （P<0.05） in D and C groups. There were no significant differences in BIS and OAA/S at each time point between the two groups （P>0.05）. Twelve cases in group D showed total amnesia and six cases showed un-total amnesia. Eighteen cases in group C showed total amnesia and nine cases showed un-total amnesia. There were significant differences between the two groups （P<0.05）. There were no significant difference between the two groups on the number of cases in application of atropine or ephedrine， incidence of respiratory inhibition， nausea， vomiting， irritability in operation and postoperative vascular crisis（P>0.05）.　Conclusions　Dex and midazolam can also provide a good sedation with low respiratory inhibition in microsurgery under regional anesthesia. They have their own advantages in anti-sympathetic nerve function， reducing bleeding， reducing myocardial oxygen consumption， sedation and amnesia.