In competitive markets, maintaining customers and building trust and loyalty are considered important in marketing relationship. Banks have invested some ways to improve customer loyalty for developing their activities. The aim of this study was to investigate the effect of relationship marketing on word-of-mouth communication in the Melli Bank branches of Roodsar in which customer loyalty had the mediation role in their relationship. The study was an applied research adopting a descriptive method. The data were gathered using questionnaire method and the study population consisted of customers of Melli Bank in Roodsar. Following this, 460 questionnaires were distributed among customers of the Melli Bank branches using convenient sampling and finally 420 questionnaires were collected for analysis. LISREL software is employed to analyze the data. Findings revealed that the relationship marketing had significant positive impact on customer loyalty and also customer loyalty led to increasing of word-of-mouth communication.