Backgruond: Dexamethasone as an adjuvant in brachial plexus block has been reported to prolong the duration of action of local anesthetic. We performed a prospective, randomised, double blind study to evaluate the effect of dexamethasone 4 mg added to local anaesthetic on the onset and duration of ultrasound guided supraclavicular brachial plexus block. Materials and Methods: 60 adult patients undergoing ultrasound­guided supraclavicular brachial plexus block were randomly divided into 2 groups of 30 each. In group1 patients received 15 ml of 2% lignocaine with adrenaline +15 ml of 0.5% bupivacaine +1 ml of normal saline (NS), in group 2 patients received 15 ml of 2% lignocaine with adrenaline +15 ml of 0.5% bupivacaine + 1 ml of dexamethasone (4 mg). The onset and duration of sensory and motor blockade in the either groups were compared. Using software package SPSS16 statistical analysis was done. Numerical variables compared by Independent samples t­test. Categorical variables compared between groups by Chi­square test. All analysis has been two tailed and p < 0.05 has been taken to be statistically significant. Results: The groups were comparable in demographic data. The onset of sensory and motor blockade in group 2 [228.33±32.386 sec; 313.33±33.767 sec] was significantly more rapid compared to group 1 [328.50±40.538 sec; 405.50±41.259 sec] [p<0.001 (HS)]. The duration of motor blockade was significantly longer in group 2 [479.83±37.312 min] compared to group 1 [325.33±36.434 min] [p<0.001(HS)]. In addition the duration of analgesia in group 2 [601.67±58.492 min] was significantly more compared to group1 [390.50±38.019 min] [p<0.001(HS)]. Conclusion: We conclude that addition of 4 mg dexamethasone as adjuvant to local anesthetics hastens the onset of sensory and motor blockade with prolongation of the duration of analgesia and motor blockade.