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This paper presents two approaches for evaluating Service Quality in a company by a customer satisfaction survey. The first approach is based on the statistical survey by the customer satisfaction measurement and the second is based on the statistical quality control method. The later method has been rarely used in such researches. The theoretical base is SERVQUAL model. In this survey three dimensions were generally defined in order to measure service quality and it was analytically considered whether it is suitable to use these dimensions or not. Then 18 factors (3 overall satisfaction factors and 15 factors which affect the overall satisfaction) were identified and also were surveyed. The data related to customers' attitudes of received services are gathered of about 30/000 customers in 15 successive months (2000 customers per month) out of statistical populations under consideration. Finally, the trends of customer's perception of these services during the survey were studied. The results of our research showed that using the feedbacks of these two approaches would cause increasing customers' satisfication and also decreasing dissatisfaction obviously.