The growing research on social media influencers suggests various key features of influencers and their post content but lacks a systematical view of their effects, including both direct and interaction effects, as marketing communication stimuli on followers’ behavioral intentions. This research applies the elaboration likelihood model to understand the key features of influencers’ marketing communication as central and peripheral factors triggering followers’ message elaboration and purchase intention. A total of 487 Instagram users participated in an online survey. The results show a difference in the impact on purchase intention between the central route and the peripheral route. Central factors (i.e., post features such as perceived originality, perceived uniqueness, and perceived informativeness) have direct effects, while peripheral factors (i.e., influencer attributes such as perceived physical attractiveness, perceived trustworthiness, and perceived self-presence) primarily work through the effect of the interaction with central factors on followers’ purchase intention. Among these factors, post uniqueness has the most prominent effect, and post informativeness has a consistent effect across varying scenarios of influencer features. These findings stress the importance of post content in influencers’ marketing communication and add to the understanding of the interaction effects of post and influencer features on influencer marketing effectiveness.