Mobile advertising is rapidly growing amid the surge in the use of smart phones in Korea. Moreover, the ad platform is moving toward the mobile application from the web site. Media engagement has emerged as an alternative choice for measuring ad effects more precisely. This study is based on the claim that product involvement is a key element in media engagement as it refers to the interaction among advertising, consumers and media environment. Media engagement is discussed in this study with the objective of investigating the effects of app engagement among smart phone users on ad performance. The results of the study revealed a significant difference between high and low app engagement groups. The high engagement group showed ad effectiveness. A significant difference in ad effects was observed between the two different levels of product involvement in the low app engagement group. The results of the study indicate the importance of app engagement and suggest valuable guidelines for the variables involved in mobile advertising such as product involvement and consumer behaviors by gender.