The purpose of this article is to increase the customer service level by expanding the method of application used by the mystery customer (MC) to resolve practical and concrete problems concerning the status of chain stores. The survival of organisations can often be dependent on their customer service level; therefore, there is an immediate need to form a permanent measurement to act as an indicator of that performance. This article concerns the application of MC methods in order to observe the level of quality service in a large retail network in Greece. Based on a quantitative methodology, the research seeks to utilize information concerning the retail operations performance and the factors that influence the evaluation. The results suggested an explicit determination of the factors of satisfied customers through the application of MC method. Action diagrams used as a performance-importance map indicate strong and weak points in terms of criteria and define the required improvement efforts. MC data showed the points or issues that service activities performance is likely to cause dissatisfaction and enables proactive measures to be taken on employee performance and classifies the improvement areas that determined by the analysis and used as a benchmarking tool.