Introduction: Use of adjuvant prolongs the brachial plexus block. Local anesthetics with various adjuvants are used to increase the duration of postoperative analgesia in supraclavicular brachial plexus block. Perineural injection of steroid is known to influence postoperative analgesia. Aim: Aim is to compare the onset and duration of analgesia of local anaesthetics with and without dexamethasone in supraclavicular brachial plexus block. Materials and Methods: Fifty adult patients undergoing below elbow surgeries of upper limb under nerve stimulator guided supraclavicular brachial plexus block were selected and randomly divided into two groups of 25 each. Group-I (Dexamethasone) patients received 36 ml of mixture of 2% lignocaine plus adrenaline (20ml), 0.5% bupivacaine (15ml) with dexamethasone 4mg (1ml). Group-II (saline) patients received 36 ml of mixture of 2% lignocaine plus adrenaline (20ml), 0.5% bupivacaine (15ml) with saline (1 ml). The onset of sensory block and duration of analgesia in two groups were compared and development of complications were observed. Study Design: Randomized Double Blind controlled study. Results: The two groups were comparable in demographic data. There was significantly faster onset of sensory blockade and prolonged duration of analgesia in the dexamethasone group than the saline group. There were no significant side effects. Conclusion: Addition of dexamethasone as an adjuvant to local anesthetics in brachial plexus block provides in significantly early-onset and markedly prolonged duration of analgesia without hemodynamic disturbances.