Background: Open-heart surgery differs from other types of surgery its need for highly skilled and talented personnel including physician and nurses as well, for this reason it is important for nurses to become familiar with the cardiac surgery. Aim of the study: This study amid to evaluate the effect of Peplau theory model application on nurses 'performance regarding care of children post open heart surgery.  Design: A quasi- experimental design. Setting: The present study was carried out in cardiothoracic surgery department and cardiac care unit in the National Heart Institute in Embaba.  Subjects: A convenience sampling of all available nurses working with children post open heart surgery (40) in previously mentioned setting and purposive sample of children (40) undergoing to post open heart surgery. Tools of data collection: Two tools of data collection were used; Tool I: A structured interview questionnaire sheet: to assess personal characteristic of nurses, personal characteristics of studied children and medical data, and assess nurses' knowledge post open-heart surgery and peplau theory. Tool II: Observation checklist to assess nurses' practice regarding post open heart surgery and discharge planning care.  Results: There was a positive correlation between total knowledge, total practices and total planning discharge care after application of Peplau theory model regarding post open heart surgery for children. Conclusion: Based on the result of the present study The level of nurses' knowledge and practice regarding care of children post open heart surgery was improved after application of Peplau theory model.   Recommendations: Implementation application of Peplau theory for nurses about care of children post open heart surgery continuously and availability printed booklets and handouts about care of children with light of peplau theory model application to help given health education for children and care givers.