Relationship marketing focuses on building long-term relationships with customers and other parties. Relationship marketing is a strategy to identify, establish and maintain to enhance relationship with customers and other stakeholders at a profit so that the objectives of all parties involved are met. The purpose of this research is to know whether relationship marketing has impact on customer loyalty at CV Matahari Engineering Medan. The methods used by the writer in this research is descriptive method and correlational method. Data used in this research are primary data and secondary data. Data analysis methods used in this research are descriptive statistic, validity test, reliability test, normality test, coefficient of correlation, coefficient of determination, linear regression analysis and Z test Based on research result, the conclusion of this research is that there is impact relationship marketing towards customer loyalty at CV Matahari Engineering Medan. The conclusion is proved with Z test. The calculation of Z test shows that Zcount (9.59) is higher than Ztable (1.96). The alternative hypothesis is accepted and null hypothesis is rejected. Coefficient of correlation is 0.861. It shows that there is strong and positive relationship between variables. Coefficient of determination is 74.2%. It shows that customer loyalty can be explained by relationship marketing in 74.2% while the remaining in 25.8% can be explained by other factors. The linear regression equation is Y = 5.594 + 1.217 X. If the relationship marketing is nil, customer loyalty be in 5.594 unit. If there is increasing 1 unit of relationship marketing, customer loyalty will increase in 1.217 unit.