Background -Neonatal hyperbilirubinemia is defined as the total serum bilirubin level more than 5 mg/dl (i.e., 86 μmol/L).[1] Phototherapy is initial treatment in neonatal indirect hyperbilrubinemia to prevent neurological damage . Hypomagnesemia has been reported as side effect of phototherapy .11Hypomagnesmia further may result in serious complications like apnea and neurological side effects.12,13 Methods - Objective:Present study aims to estimate the serum Mg++ in the term neonates with increased amount of bilirubin in the blood and to compare pre-phototherapy and post-phototherapy serum magnesium level. Methodology : A prospective hospital based cohort study was conducted on 70 eligible neonates admitted in department of Pediatrics in a tertiary health care institute in North India, conducted from January 2023 to October 2024. After approval of ethical committee, informed consent was obtained from parents of eligible neonates. Serum bilirubin and magnesium measured pre-phototherapy and post-phototherapy to study the effects of phototherapy on serum magnesium. Results–Our study included 100 neonates with hyperbilirubinemia who received phototherapy, 40 (57.5%) neonates were male and 30 (42.9%) were female. Mean Gestational age was 37.7 +/- 0.84 weeks. Mean birth weight was 2.68 +/- 0.39 kg. 43 (61.42%) neonates were delivered by LSCS and 27 ( 38.33%) were delivered by NVD. Mean neonatal age was 57.24 +/- 19.96 hours at the time of initiation of phototherapy.The mean pre-phototherapy and Post-Phototherapy magnesium level were 2.16 +/-0.26and1.95 +/- 0.34 respectively and magnesium decreased significantly after 24 hours of phototherapy p value is 0.0001 . No significant correlation was seen while correlating pre-Phototherapy total serum bilirubin and pre-Phototherapy serum magnesium levels. Conclusion–Serum Magnesium decreased in term babies significantly following 24 hours double surface phototherapy irrespective of gender and birth weight group. Also no co-relation between pre-phototherapy serum magnesium and serum bilirubin level was observed.