Building customer loyalty can not be easily achieved, but it requires a long process. One of the ways is with service quality provided to the consumer in order to encourage consumers to be loyal to the company’s product and service. The objective of this study is to know the impacts of service quality on customer loyalty at PT Indoteras Sumatera Medan. The methods used by the writer in this research are descriptive method and correlational method. Data used in this research is 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 service quality has a significant influence on customer loyalty in PT Indoteras Sumatera Medan. Z test give result that Zcount (9.89) is higher than Ztable (1.96).Correlation value in 0.878 shows that there is strong and positive relationship between service quality and customer loyalty in PT Indoteras Sumatera Medan. Coefficient of determination value in 77.1% shows that the service quality has a contribution on change of customer loyalty in amount 77.1% while the remaining in 22.9% is contributed by other factors. The linear regression equation is Y = 6.794 + 0.625 X. Coefficient of regression in 0.625 shows that increasing of 1 unit service quality can make the increasing of customer loyalty in 0.625 unit.