Influencer marketing industry is growing every year. However, not much research has been done on influencer marketing relative to its increasing market size. This research aims to find the existence, duration, and mechanism of influencer marketing effects by calculating its elasticity and ROI comparing with online advertising. I use data from a major Korean cosmetic retailer, which contains one product of a global cosmetic brand. Both elasticity and ROI of influencer marketing are greater than those of online advertising. Moreover, this research enables us to comprehend the mechanism through which influencer marketing affects sales. Influencer marketing not only directly affects sales but also indirectly increases online sales by increasing eWOM volume. This indirect effect is significant only in online channel. These findings offer guidance to managers who consider allocating budget to influencer marketing.