The main purpose of this paper is to review the application of queuing theory and to evaluate the
parameters involved in the service unit for the sales checkout operation in ICA Supermarket. Therefore, a mathematical model is developed to analyze the performance of the checking out service unit. Two important results need to be known from the data collected in the supermarket by the mathematical model: one is the ‘service rate’ provided to the customers during the checking out process, and the other is the gaps between the arrival times (interarrival time) of each customer per hour. In order to get an overall perspective of the customer’s quality of service, the questionnaires which indicate the result in percentages, are also used to get the evaluation from the customers directly.