Background: Interscalene brachial plexus block is usually associated with sparing of C8, T1 dermatomes and ulnar nerve. This study have compared the effect of adding magnesium sulphate with bupivacaine on the blockade of individual dermatomes in interscalene block. Materials and Methods: 60 patients were randomly divided into two Groups. Interscalene block was given to all the patients. Group 1 received 20 ml of 0.5% bupivacaine with 0.5 ml of 50% MgSO4 (Total volume = 20.5 ml). Group 2 received 20 ml of 0.5% bupivacaine with 0.5 ml of normal saline (Total volume = 20.5 ml). Block was evaluated every 3 minutes till 15 minutes after the injection of local anesthetic or till minimum Grade 3 sensory and motor blockade was achieved. Results: In our study, the mean time of onset of sensory and motor blockade (achievement of minimum Grade 2 blockade) was comparable between the two Groups. The mean time of onset of complete sensory blockade (achievement of minimum Grade 3 sensory blockade) was also comparable between the two Groups (p > 0.05). Grade 3 sensory blockade to C8 and T1 dermatomes was achieved in 70% of patients (n = 21) in Group I and 23.3% of patients (n = 7) in Group II at 15 minutes and the difference was significant (p = 0.001). Grade 3 motor blockade (by Modified Bromage Scale) was achieved in 70% of patients (n = 21) in Group I and 23.3% of patients (n = 7) in Group II at 15 minutes which was statistically significant (p = 0.001). Conclusion: Magnesium sulphate as an adjuvant tobupivacaine in interscalene block decreases the incidence of C8 and T1 dermatomal sparing and increases the chances of complete motor blockade.