Background: One of the most important nursing functions is to administer medications safely. Effective and safe administration of medication is one of the most common nursing activities that require a set of knowledge and specialized skills. Aim of the study: Evaluate the effect of training program on reduction of nurse's medication errors. Subjects & Methods: Research Design: quasi-experimental design was used in the current study. Setting: study was conducted at Intensive Care Units at specialized medical hospital in Mansoura University which include three intensive care units namely (cardiac, hepatic and diabetic). Subject: study subjects include 50 nurses working in above mentioned setting, all patients assigned to observed nurses and drug doses that administer to the patient (n=230). Tools of data collection: two tools were used, 1) questionnaire interview sheet to assess nurses' knowledge about safe medication administration practice and medication errors. 2) Observation check list to assess medication administration practice for detecting medication errors there were used before and after implementation of the program. Results of the present study revealed that only 18% of nurses had satisfactory knowledge about medication administration and medication errors this percentage improve to 90% immediately after program implementation & 84% in the three months later and total high errors was 70.5% in pre program this percent decrease to 24.3% in three months later phase. Conclusions: Nurse's knowledge were improved immediately after program and three months later. As regard to medication errors, the percent of total high errors was decreased in comparing to preprogram implementation. Recommendations: Implementing of regular or scheduling training for all nurses in hospitals to improve and refreshing their knowledge and skills. Implement double check system to prevent medication administration errors and improve the effectiveness of communication among health care providers to ensure continuity of patient care and reduce medication errors.