The rapid growth of cosmetic industry give an opportunity for companies to acquire and engage consumers by using a new e-commerce paradigm called social commerce (s- commerce). One of the most prominent s-commerce among Gen Y is Instagram as this social networking site enables users to browse online stores through its visual interface via mobile device. They are also used to online shop through this platform. Recent researches have shown interests in investigating factors that affect the online shopping behavior in social networking sites but very little is done to understand the process of online buying decision, notably Instagram. In this study, the data were collected by interviewing four participants who have experienced the buying of lip product through Instagram. Online buying decision process framework by Karimi (2013) was adapted for this purpose. The interview was carefully transcribed and analysed using qualitative content analysis (Mayring, 2000). The result of this study shows that the process of online buying decision process on Instagram differs from e-commerce or other social networking sites (SNSs), in terms of how they function this SNS as a platform to search and make buying decision. Major factors affecting its process are habitual Instagram usage and trust to Instagram storeowners. The result also signifies consumers impulse buying tendency. As there is no adequate research on this phenomenon, consumer behavior implications and further research directions are discussed in this study. Keywords: Online Buying Decision, Instagram, Gen Y