This study is a randomized , triple blinded, parallel group placebo controlled trial to determine the role of magnesium sulfate in perinatal asphyxia. This study was conducted in fifty term neonates with perinatal asphyxia, who were selected in accordance with the laid down criteria. Neonates were assigned randomly to receive either 3 doses of magnesium sulfate infusion at 250 mg/kg/dose 24 hour apart (treatment group) or 3 doses of normal saline infusion at 1 ml/kg/dose 24 hour apart (placebo group). Calmagite method was used for estimation of serum magnesium sulfate levels. Distribution of neonatal baseline characteristics and severity of hypoxic ischemic encephalopathy were statistically similar in treatment and placebo groups. The mean serum magnesium concentration in the treatment group remained >1.4mEq/L in initial 72 hours. In our study, postnatal magnesium sulfate infusion was found neuroprotective in moderate encephalopathy, which was reflected by early control of seizures (p=0.04), early appearance of normal cry (p=0.02), early appearance of normal activity (p=0.02), early acceptance of full oral feed by sucking (p=0.04), good short term outcome (p=0.04). Postnatal magnesium sulfate infusion was not found neuroprotective in severe encephalopathy (p=>0.05). No significant differences were observed in blood pressure between both the groups. Finally, this was concluded that postnatal magnesium sulfate therapy causes fevorable neurological outcome as primary outcome and not causing hypotension at given dose as secondary outcome.