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Background and Objective: Occurrence of nursing error is important issue in patients safety. This study was done to determine the role of workload, sleep, mental health and individual factors in occurrence of nursing errors. Materials and Methods: This descriptive study was done on 268 nurses who employed in hospitals of Tehran University during 2011. Data were collected by demographical information questionnaire along with work environment conditions, Global Sleep Assessment Questionnaire (GSAQ), Pittsburgh Sleep Symptom Questionnaire (PSSQ) and Mental Health Inventory (MHI-28). Results: Sleep distruptions (OR=3.260, P<0.002), sleep behaviors (OR=2.946, P<0.003), psychological distress (OR=2.530, P<0.006), psychological well-being (OR=0.775, P<0.002), and work overload (OR=5.240, P<0.009) significantly able to predict occurrence of nursing errors (P<0.05). Conclusion: Work overload, sleep distruptions, sleep behaviors, psychological distress and psychological well-being cooperatively were resulting in occurrence of nursing errors.