The emergency department as entrance of hospital and in close relationship with other sections of the emergency service providers provides the wide range of services from the stage of acceptance and initial assessment to telling what to do and release of patients and damaged. In recent years, improving the performance of hospitals’ emergency department has been included as an important goal in health policy-making of different countries. Since the emergency department has a complex structure and several factors affect its performance, identifying these effective factors is very important for improving the performance of the emergency department. The aim of this research was to determine the factors affecting the improvement of the performance of the hospitals’ emergency department in the world. Using MESH and free text, we sought and retrieved the relevant articles from the most appropriate medical databases (the Cochrane Library, PubMed and Scopus) through three separate search strategies up to July 2015. The inclusion criteria were as follows: 1) Studies with specific influence factors; 2) Articles written in English; 3) Those articles conducted in compliance with emergency department performance improvement. Data were analyzed qualitatively using a thematic synthesis technique. After screening the retrieved papers via PRISMA framework, from the 1473 papers, 25 studies were included in the final phase. Criteria for selecting performance improvement (in pre assessment and in the assessment phase) were categorized into five main themes: Communication, Team work, Clinical path way, Patient flow, and Other factors. “mutual respect” had the maximum frequency in communication (4 studies); “group behavior” and “collaboration” had the maximum frequency in team work (7 studies); “time” had the maximum frequency in clinical path way (3 studies); “services” had the maximum frequency in patient flow (5 studies); and “patient safety” had the maximum frequency in other factors (10 studies). The results showed that emergency department is unique in terms of workload and need to be careful of environment and identifying the factors affecting the performance of the emergency department at the increase of the efficiency of hospital is of particular importance. Also redesigning the processes of improving the performance an have an important role in increasing the quality of providing services to the hospitalized patients in the emergency department.