Relationship marketing, which focuses on the relationship with the customers, is considered important in the field of service marketing. This research explores the relations of customers' perceived expertise of service provider and customer's sociability, and how these factors are related to the intention to use. Relationship quality is set as a mediating variable. The relationship quality is measured by trust and commitment. Also, the relationship between trust and commitment is measured. The result of the analysis of survey data shows that, expertise and sociability factors have positive effects on relationship quality. Also, the two factors of relationship quality, trust and commitment, have positive effects on the intention to use. In addition, trust is positively related to commitment. In conclusion, service provider's expertise and customers' sociability are the factors of IT service's success.