Purpose -The purpose of this paper is to investigate the effects of internal marketing on service quality and the mediating role of OCB in their relationship. Design/methodology/approach -A total of 162 random samples of the customers and employees of the Greater Tehran Gas Company were taken. The applied research methodology is based on survey design techniques, specifically structural equations modelling. To test the model questionnaire was designed, applied and then analyzed. Findings -Findings clearly demonstrate that internal marketing actions can have a direct, meaningful and positive effect on organizational citizenship behaviours. Furthermore, the conceptual model shows that organizational citizenship behaviours act as a mediator between internal marketing and service quality.Research limitations/implications -The present study does have a number of methodological limitations that suggest areas for future research. First, the sample was from a public company, and this unique setting may limit the external validity of the findings. Second, the country in which the authors tested the hypothesis is an Islamic country with different cultural values from western countries and this fact might limit the generalization of the findings. But the authors believe that the proposed model and results of this research can aid service organization to offer better service quality to both their internal and external customers. Originality/value -The paper may be of high value to researchers and managers in Marketing and Human Resource Management scopes. Results of this research have valuable guideline that can have tangible outcome for organization.