Background: In recent decades, the increasing crowdedness of the emergency departments has posed various problems for patients and healthcare systems worldwide. These problems include prolonged hospital stay, patient dissatisfaction and nurse burnout or job dissatisfaction. Objectives: The aim of this study was to investigate the effect of emergency department (ED) nurses’ job specialization on their job satisfaction and the length of patient stay in the ED. Patients and Methods: This before-after quasi-experimental study was conducted from April to May 2014 at the Baqiyatallah Hospital, Tehran, Iran. Initially, 35 patients were recruited as controls and the length of their stay in the ED was measured in minutes via a chronometer; Moreover, nurses’ job satisfaction was evaluated using the Mohrman-Cooke-Mohrman job satisfaction scale. Then, a job specialization intervention was developed based on the stabilization model. After that, 35 new patients were recruited to the treatment group and received specialized care services. Accordingly, the length of their stay in the ED was measured. Moreover, the same nurses’ job satisfaction was re-evaluated after the study. The study intervention lasted one month. Data were analyzed using the SPSS software version 20 and statistical tests such as the Kolmogrov-Smirnov, the paired and the independent t, and chi-square tests. Results: There was a significant difference between the two groups of patients concerning the length of their stay in the ED (P < 0.001). Moreover, compared with the pretest readings, nurses had greater job satisfaction after the study (P < 0.001). Conclusions: The job specialization intervention can improve nurses’ satisfaction and relieve the crowdedness of the EDs.