Medication errors can be a serious problem that exposes patients to preventable risks. The present study was aiming at investigating the effect of training in reduction of medication errors, through developing, implementing, and evaluating an in-service training program for reduction of nurses medication errors. The study was carried out in Assiut University Hospitals, including all general medical and surgical units and medical and surgical intensive care units. A total of 67 nurses were selected by a systematic random sampling technique from those 200 nurses included in the needs assessment who are selected in the previous master degree related to medication errors. Four different tools were used to collect study data. These were a knowledge test, chart audit, an observation checklist, and an attitude scale. The tools were applied 3 times, before implementation of the training program, immediately after implementation, and after three months from the post test. The result denoted that there were marked improvement in nurses' knowledge, attitude, and practice immediately after implementation of the training training program. This improvement was mostly retained after 3- months, with slight decline, particularly related to attitude, although still higher compared to pre-implementation level. However, the training program had no effect on nurse's documentation of medication errors, which could be attributed to the absence of the medication errors record in the hospital. Strong correlations were revealed among nurses' knowledge, attitude, documentation, and performance scores. On the light of the present findings the authors recommended the necessity of improvement of medication education at all levels is urgently needed, ~etting standard for medication administration process by nurses, Setting policies regarding medication error reporting, Simplification of the steps of transcription is recommended.