Many firms have been collaborating with social media influencers in promoting their brands as they have intense influential power in affecting consumers’ purchasing decisions. However, after famous YouTubers have turned out to be engaging in undisclosed ads, people started to cast doubt on the effectiveness of influencer marketing. The purpose of this present study is to investigate the underlying mechanism through which the YouTuber's immoral behavior such as undisclosed ads negatively affects attitudes of consumers toward the brands they have endorsed. YouTube platform enables people to actively interact with YouTubers so that they can form close relationships with YouTubers. Engagement in undisclosed ads is an immoral behavior that breaks trust of subscribers, which in turn makes them feel betrayed and subsequently reduces the credibility of contents provided from the YouTubers. We also expect that low information credibility negatively affects brand attitude. Finally, we predict that the mediating effect through perceived betrayal and perceived information credibility will be stronger when people believe that YouTubers deserve their fame. Data from the experiment using the scenario supports all Hypotheses. The current finding contributes to existing research on celebrities’ immoral behavior in that we examine both emotional and cognitive responses to their transgression while previous studies have primarily focused on cognitive responses such as justification.