Ｏｂｊｅｃｔｉｖｅ　To evaluate the effects of dexmedetomidine(Dex) on BIS of electroencephalograph in patients received supraclavicular brachial plexus blocking with levobupivacaine under ultrasound-assisted guidance.　Methods　Eighty patients undergoing distal arm and forearm surgeries and classified into ASA classⅠorⅡ were divided into two groups. One was control group (C group), in which patients received supraclavicular brachial plexus blocking with 30 ml levobupivacaine and 1 ml normal saline. The other was Dex group, in which patients(D group) received supraclavicular brachial plexus blocking with 30 ml levobupivacaine and 100 μg Dex in 1 ml normal saline. The supraclavicular brachial plexus blocking was performed under the guidance of ultrasound. BIS value, Ramsay sedation scores, and time to first rescuing analgesic after the surgeries(TAR) were recorded.　Results　In D group, the BIS values decreased significantly until 20 min after the brachial plexus blocking (P＜0.05), but the Ramsay sedation scores was significantly increased(P＜0.05), in comparison with C group. Time to administration of the first rescuing analgesic after the surgeries for pain relief was significantly delayed in D group than in C group(P＜0.05).　Conclusions　Perineural dexmedetomidine administration for supraclavicular brachial plexus blocking with levobupivacaine had sedative effects that correspond to drops of BIS values, and significantly prolonged the duration of postoperative analgesia.