An increasing number of retail websites (e.g., Amazon) and online communities (e.g., Shopping.com) have implemented Web 2.0 to provide user reviews for consumers to make purchase decisions. Current IS and marketing literatures show that user reviews can form powerful social influence on consumers. However, few studies systematically examined how social influence is developed and its impacts. To bridge the gap, we propose that review quality, review consistency and social presence impact the formation of two types of social influence: informational and value-expressive influences, which ultimately determine consumers’ perceived decision quality and perceived usefulness of the system. This study will generate managerial insights on online marketing and system design in E-commerce.