Abstract: Background: Hypoxic Ischemic Encephalopathy (HIE) is a leading cause of neurological impairment and newborn death, accounting for more than a million neonatal deaths globally each year. The condition requires prompt medical intervention and one of the therapeutic approaches is cooling therapy. Purpose: To assess the effect of on-the-job training on neonatal nurses' performance related to cooling therapy for HIE. Method: A quasi-experimental research design was utilized. A convenient sample of 47 nurses at the Neonatal Intensive Care Unit (NICU) affiliated to Mansoura University Children's Hospital (MUCH) was included. Neonatal Nurses' Structured Questionnaire, Nurses' Practices Observation Checklist, and Nurses' Attitude Scale were used to collect the data. Results:  The studied neonatal nurses' who had satisfactory knowledge regarding HIE and cooling therapy increased from more than one-third pre-intervention to the vast majority post-intervention. Furthermore, pre-intervention, more than two-fifths of the surveyed neonatal nurses demonstrated competent practices, compared to the vast majority of them post-intervention. Also, more than two-thirds of studied neonatal nurses had negative attitudes toward cooling therapy for HIE pre-intervention, while the majority of them had positive attitudes post-intervention. Conclusion: The comprehensive on-the-job training program demonstrated significant positive outcomes across multiple dimensions within the nursing cohort. The findings reveal a substantial improvement in neonatal nurses' knowledge, attitudes, and practical competencies related to HIE and cooling therapy following the intervention. Recommendations: On-the-job training programs related to cooling therapy for HIE should be conducted for nurses in NICUs.