Aims and Objectives: To compare the duration of post operative analgesia with different doses of magnesium sulphate as an adjuvant in USG guided supraclavicular brachial plexus block and its side effects. Material & methods: Ninety patients aged 18-50 yr ASA Gr 1-2 divided into 3 groups of 30 each undergoing upper limb surgery under USG guided supraclavicular brachial plexus block. Group C (control group)- (n=30) received 20 ml of 0.5% Bupivacaine + 5 ml of normal saline (NS). Study group 1 (S1) - (n=30) received 20 ml of 0.5% Bupivacaine + 4 ml of NS + 100 mg (1ml) of magnesium sulfate. Study group 2 (S2) - (n=30) received 20 ml of 0.5% + Bupivacaine + 3 ml of NS & 2 ml (200 mg) of magnesium sulphate. Results: Onset of sensory block in Group S2 (6.5±1 min), in S1 (10±2.8 min) and in C (15±3 min). Onset of motor block in S2 (9±2 min), in S1 (13±2.2 min) and in C (19±2 min). Duration of post-operative analgesia in S2 (540±25 min), in S1 group, (440±20 min) and in control group C (200±15 min). Addition of MgSO4 as adjuvant hastened the onset of sensory and motor block in study group as compared to control. Duration of sensory and motor block were more in group S2 as compared to S1 and C. Duration of postoperative analgesia was significantly prolonged in group S2 as compared to S1 and C (p<0.001) without increased incidence of side effects. Conclusions: Addition of magnesium sulphate to local anesthetics in brachial plexus block prolongs the duration of postoperative analgesia. It is dose related, 200mg has greater efficacy than 100mg without increased side effects.