Introduction: The quality of service is a key ingredient in the success of health care organizations. Therefore several tools have been developed to measurement of service quality but the Parasuraman 22 item SERVQUAL instrument has been created based on the gap model remains the most widely used in a variety of industries. This study attempts to replicate the SERVQUAL measurement of Parasuraman et al in determining the existence of service gap in Shahid Rahnemoon hospital. Materials and Methods: this descriptive - applied study was carried out through cross-section of method in 2010. The research population comprised of patients admitted to Shahid Rahnemoon hospital words. 61 patients were calculated by sample size formula for unlimited population and obtained randomly. The data was collected by 22-item SERVQUAL questionnaire that was designed according to gap model. Data analysis was carried out through SPSS and LISREL -software. And Kolmosrof - Smirnoff and paired sample t -test. Results: the results demonstrated that there was a quality gap in Shahid Rahnemoon hospital. In the other hand, the results showed that there were significant differences between expectations and perceptions of patients in SERVQUAL dimensions. Also, the study identified some critical and problematic items that have main role in constructing observed quality gap. Conclusion: the existing quality gap means patients expectations exceed their. Perceptions. thus, improvement are needed across SERVQUAL dimensions.