The present research was conducted in a bank in Tehran, Iran, in 2009/2010.  This report is the outcome of a field research, which aimed to determine the quality of services offered by Sepah Bank, and also to study the relationship between the service quality, satisfaction and loyalty. In this research, the service quality standard model has been used for evaluation of service quality, Gremler and Brown (1996) model with some revision was used for evaluating the loyalty, and the instrument offered by Bitner and Hubbert (1994) was used for evaluation of customer satisfaction. The focus of this research is a Sepah Bank branch around Fatemi St., Tehran, Iran, and 147 customers of this bank were sampled. The results of this research show that in all aspects, customers’ expectation, are higher than their perceptions of the Bank’s operation, and in fact the quality of offered services is low. Besides, this research findings show that the customer satisfaction plays the role of a mediator in the effects of service quality on service loyalty. These findings are further explored.