Background: Recently, considerable attention has been paid to medical errors in health care system. Taking into account that nurses spend more time with patients in comparison with other staff, they are more prone to human errors. The present research conducted to investigate nurses' errors in delivery emergency ward in a hospital affiliated to Tehran University of Medical Sciences.

Material and Methods: In the present coss-sectional study,at first, task analysis was conducted  to nursing proffession using Hierarchical Task Analysis (HTA) technique by FGD. In next step, the Human Error Probability (HEP) was determined utilizing SPAR-H method and dependences of performance-shaping factors (PSFs) and action and diagnosis activities were identified. Finally, after determinig dependency level, the final diagnosis HEP was calculated and risk assessment level was carried out  using the risks' probability and intensity tables.

Results: According to the results, four groups of studied tasks, the lowest errors were belonged to "working with serum pumping machine'' subtask with the error level of 0.055 and the highest error probability related to '' transfusion of blood products'' subtask with the error rate of 0.78. Moreover, special responsibilities of the wards had the highest level of undesirable risks.

Conclusion: In the present study, the analysis of identified errors reveals due to high work demand, insufficient time and the need for accurate administrative monitoring and providing required arrangements, the main causes of errors can be attributed to high level of stress and complexity in the tasks of delivery emergency ward.