This study examines the effects of SNS fashion influencer's authenticity on the followers' relationship maintenance intention and product purchase intention through the mediating role of fanship. In addition, influencer's attractiveness is proposed as a moderator affecting the authenticity on fanship; in addition, authenticity is operationalized as honesty and expertise. A written survey is developed for 181 female Instagram users. The analyses of the data yields the following findings. First, honesty has a positive effect on fanship, relationship maintenance intention and purchase intention, while expertise affects fanship and relationship maintenance intention, but does not have a significant effect on purchase intention. Second, honesty and expertise have a positive effect on follower behavior intention through the partial mediation of fanship. Third, social attractiveness and physical attractiveness have a significant effect on fanship. However, all interaction effects between authenticity and attractiveness are not significant. Unlike authenticity, attractiveness has no direct effect on follower behavior intention. This study verifies the critical role of authenticity for an influencer in the context of SNS marketing and commerce, as this factor leads to followers behavioral intention as well as fanship in both direct and mediated paths.