This study aimed to assess the quality of four branches of social security organization in Tehran Province, using SERVQUAL model and assessing the gaps between expectations and perceptions of customers in each branch of the service quality dimensions (factors tangible, reliability, responsiveness, assurance, empathy and diversity). The population study was the daily costumers of four provincial branches, each group 120, total 480 people. Sampling was simple randomize and the study was cross sectional. To conduct this project, using SERVQUAL standard questionnaire included questions on both the expectations and perceptions of twenty-four 7-point Likert scale were prepared. The results were analyzed with SPSS software version 21. Validity and reliability of the questionnaires were confirmed (cronbach coefficient=0.87). Results showed considerable difference between expectation and perceptions in all service quality dimensions (Pval<0.0001). Generally, customers have expectations beyond the services available at the branch offices and branches of social security organization have failed to meet the expectations of their clients.