The objectives of the loyalty programs are to encourage buyers to repurchase, increasing customer’s satisfaction and make them not easily influenced by other offers from competitors. Customer behavior depends on the loyalty program of the company. The objectives of this research is to find out the influence of loyalty program on customer behavior at PT. Arianto Darmawan Medan. PT. Arianto Darmawan is presently the only one of its kind in Indonesia and South East Asia that offers such a wide-ranging of quality products.The methods used by the writer in this research is the descriptive method and causal study. Population of this research is customer of PT Arianto Darmawan Medan is 162 customers and sample in this research will be 115 customers. The result of coefficient of determination is 0.496, which means 49.6 percent of customer behavior at PT Arianto Darmawan Medan is effected by loyalty program. Based on the Z test result, Z-count (7.56) is higher than Z- table (1.96), therefore alternate hypothesis is accepted. So, there is an influence of loyalty program on customer behavior.Based on the research results of research of the company, the recommendations of this research would be improve the loyalty program by paying attention to the customers feeling of their status, Then, the company also need to improve the loyalty program by giving identification to the old customers. In addition, the company must have any competitive advantage and company also must give more benefits for those who have used their products for long time.