Today, the importance of customer loyalty is increasing more and more, and is being recognized as the most important competitive advantage of organizations and its role in success and development of business is undeniable. Purpose of this study is to investigate the effect of Investing in relational marketing on customer loyalty. The research method was descriptive-survey and an applied one. The statistical population of the study includes customers of Pars Khazar`s permanent sale branches in Tehran. The sampling method was Non-random sampling available. Using Cochran’s formula 384 people were chosen as the sample size. To gather the data two standard questionnaires, namely Lu and Lin (2010) for Relational Marketing, and Randell Till (2005)  to measure Customer Loyalty were used. Univariate regression analysis test was used to analyze the data. Descriptive and inferential statistics (Kolmogorov test, correlation coefficients and regression) and Spss software were used for data analysis. The results indicated that Investing in Relational Marketing has an impact on customer`s loyalty.