As online social network are so popular, their users are getting used to make decisions based on opinions collected from their friends. While conven tional decision support system has been extensively investigated, little specific mechanism, however, on how social networks can help users with online purchasing decision-making is developed. By introducing design rationale and social impact theory into system development, we used information technol ogy as tools to design a social network-based decision support system framework for consumer purchase decision problems. QOC schema was used to describe the reasoning process of possible product a lternatives. Further, social impact was used to select the decision group members and measure the effect of changing decision members’ attitude toward a specific options or criteria. Our empirica l study further showed that the proposed framework can perform better than benchmark methods.