Worldwide, rising number of patients and overcrowding in Emergency Department (ED) is considered as a major concern in most of hospitals. Different studies showed that overcrowding affects waiting time, length of stay, patients' satisfaction and mortality rates. Moreover, long waiting time can be associated with delayed care and poor outcomes. In Oman, informal triage system is used in most of secondary healthcare centers SHCs. No available data on the effectiveness of implementing formal triage system in primary health care centers in Oman. Purpose: To assess the effectiveness of implementing Emergency Severity Index (ESI) triage system in a primary health care center in Oman in terms of waiting time, length of stay (LOS), patient satisfaction, and patient safety. Method: This study is a pre-post quasi-experimental design and conducted at a PHCC in Muscat, Oman. A convenience sample of 187 patients before implementing ESI and 102 patients after implementation were included in this study. Data were collected using the computerized medical record system for registration time, triage time, and time for receiving the treatment. Brief Emergency Department Patient Satisfaction Scale was used to measure patient satisfaction. Inter-rater agreement in accuracy of triage level between triage nurses and ESI expert used to indicate patient safety. Result: The total patients approached were 410 patients and the response rate in this study was 70.5%. The mean time from registration to triage was reduced, (M = 0:23, SD = 0:19) for pre ESI implementation, compared to (M= 0:18, SD = 0:14) for post implementation, (p = 0.01). The mean LOS was reduced, for pre implementation (M = 1:24, SD = 0:41), compared to (M= 1:09, SD = 0:37) for post implementation, (p=0.002). However, the mean of patient satisfaction in the pre implementation (M = 65.01, SD = 8.73) was slight lower than mean of patient satisfaction in the post implementation (M= 66.95, SD = 8.33, p = 0.07). The inter-rater agreement of triage level in post ESI implementation showed marked improvement in accuracy of decisions with almost perfect (Cohen's Kappa = 0.910, p<0.001), compared to the pre implementation (Cohen's Kappa = 0.082, p=0.005). Conclusion: Although the results from the current study are from a single setting, but it demonstrated that ESI system can contribute to decrease the negative overcrowding outcomes in PHCCs in Oman. Key words: Primary Health Care Center (PHCC), Emergency Severity Index (ESI), waiting time, length of stay (LOS), patient satisfaction, patient safety.