Background: Hyperbilirubinemia is one of the most prevalent problems in neonates. Jaundice is observed during first week of life in approximately 60% of term neonates and 80 % of preterm neonates. Phototherapy is one of the routine methods for management of hyperbilirubinemia. Many studies reported that serum levels of calcium and magnesium are significantly affected after 48 hours of exposure to phototherapy. Objectives: The aim of the work was to study the effects of phototherapy on serum calcium and serum magnesium in full term and preterm neonates with hyperbiliurbinemia. Patients and Methods: The current study was prospective cohort that was conducted in neonatal intensive care unit (NICU), Zagazig University children hospital in period between August 2018 to July 2019. 60 jaundiced infants admitted to (NICU) in the study. They were divided into 2 groups; Group I: 43 patients (full terms), Group II: 17 patients (preterms). Patients were examined for the effect of phototherapy on serum level of calcium and magnesium. Results: There was a statistically significant difference between serum calcium and serum magnesium before and after 48 hours of exposure to phototherapy (P <0.001). Calcium was distributed pre and post phototherapy as 9.63±0.79 and 9.04±0.78 respectively and Magnesium was distributed pre and post phototherapy as 2.17±0.23 and 1.91±0.18. Total bilirubin was distributed pre and post phototherapy as 16.76±2.99 and 11.0±2.45 respectively and direct bilirubin pre and post as 1.05±0.33 and 1.05±0.32 respectively. Preterm were 28.3% (17 in number) and fullterm were 71.7 % (43 in number). Conclusion: Based on the findings of our study, phototherapy can significantly decrease the levels of calcium and magnesium in jaundiced term and preterm infants undergoing phototherapy.