Introduction: Perinatal asphyxia (PA) or neonatal hypoxic ischemic encephalalopathy (HIE) is an insult to the foetus due to temporary interruption of oxygen availability to various organs. It is one of the leading causes of neonatal death and major attribute to neuromotar disability. Objective: To determine effectiveness of Magnesium Sulphate infusion in full term neonate suffering from moderate and severe perinatal asphyxia. Methods: The present study was conducted in the Dept. of Pediatrics, Faridpur Medical College Hospital, Faridpur, Bangladesh from January 2020 to December 2020. The study was approved by the hospital ethics committee beforehand. All full term neonates admitted within 6 hour of birth with moderate or severe perinatal asphyxia were included in study. Full term neonates with perinatal asphyxia were selected and classified according to criteria laid by Sarnat and Sarnat. After recording baseline characteristics they were randomly assigned to receive either 3 doses of magnesium sulfate infusion at 250 mg/kg/dose 24 hours apart (treatment group) or 3 doses of normal saline infusion at 1 ml/kg/dose 24 hours apart (placebo group). Neurological assessment was performed at time of admission and at fourteenth day. Distributions of perinatal factors, neonatal baseline characteristics, and mean time for initial infusions were noted in pre designed proforma and analyzed statistically. Results: Distribution of neonatal baseline characteristics and severity of hypoxic- ischemic encephalopathy were similar in treatment and placebo groups. 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). Postnatal magnesium sulfate infusion was not found neuroprotective in severe encephalopathy (p=>0.05). No significant differences were observed in the cranial sonography and electroencephalography between treated and placebo groups (p=>0.05). No significant differences were observed in blood pressure, heart rate and respiratory rate between both the groups. Conclusion: Postnatal magnesium sulfate therapy may result in early recovery and favorable neurological outcome in term neonates with moderate encephalopathy.