Background: The mean emergency department Length of stay is an important indicator of the effectiveness of health services. It can be affected by several patient-related factors including gender, age, time of visit, main complaint, and allocated triage level. This study evaluated the relationships between these factors and emergency department Length of stay.Methods: The present descriptive-analytical study applied convenience sampling to recruit 408 adult patients who presented at the emergency department of Besat Hospital (Hamadan, Iran) during a three-month period in 2013. A chronometer was used to measure the time interval between the patients’ presentation at the emergency department and their discharge (i.e. Length of stay in the emergency department) through direct observation. The determined values were recorded in a relevant form. The Emergency Severity Index (ESI) form was also completed by triage nurses in charge. Analysis of variance (ANOVA), Kruskal-Wallis and independent-t tests, were performed to analyze the data. All analyses were conducted in SPSS/16 at a confidence interval of 95%.Result: The mean emergency department Length of stay was 133.26±41.91 minutes. There were no significant relationships between Length of stay and the patients’ gender (P=0.52), referral type (P=0.14), and history of hospitalization (P=0.80). However, Length of stay was significantly related with the patients’ age, time of admission, and main complaint (P<0.001).Conclusion: Considering the role of patients’ age, time of visit, main complaint, and level of triage in determining emergency department Length of stay, these factors should be incorporated as indicators of effectiveness of healthcare centers.