The study describes the design and implementation of the data mining applications for customer relations management in order to improve the marketing operation in one Internet Service Provider (ISP) company. The ISP business has become one of the most fiercely competitive sectors of the service industry. In such a competitive environment, traditional customer relations strategies of marketing department deployed to fuel business growth become less effective. The study therefore proposes two applications, customer segmentation and attrition analysis, to improve customer retention. The purpose of customer segmentation is to define new business opportunities in order to reduce the cost of marketing campaign. The purpose of the attrition model is to identify the profitable customers who are likely to drop their service, and thus improve customer retention.