The aim of this study was to investigate and to compare customers' loyalty in private and public sport clubs of Tehran with a descriptive survey and field method. The statistical population of the study included customers of sport clubs in Tehran who had participated in sport activities for at least 6 months. Based on cluster sampling method, 4 regions out of 22 regions from Tehran were randomly selected; then, 10 private clubs and 10 public clubs from these regions were selected; then, 200 subjects from each club were randomly selected. A researcher–made questionnaire with 0.88 reliability was used to collect data. 400 questionnaires were distributed among the subjects and finally 386 questionnaires were collected. Data were analyzed with factor analysis and independent sample t test. The results showed no significant difference in brand selection, change costs, satisfaction and loyalty from customers' viewpoint (P>0.05), but a significant difference was observed in service quality between private and public sport clubs (P˂0.05). It can be concluded that the most important factor in customers' loyalty was service quality.