With the prosperity of the social media, online social interaction has played an increasingly important and evolutionary role in personal interactions. Borrowing Turner’s composite model of social interaction as the theoretical framework, this study examines the effects of three sets of factors, including characteristics of individual users, characteristics of dyadic friendship, and characteristics of network structure, on the likelihood of the friendship formation. This study finds that the network structure effects (i.e., the triangular closure process, connectedness of personal network, and degree similarity) dominate tie formation, in comparison with the dyadic characteristics and the actors’ characteristics. Moreover, this study found that “preferential attachment” is not applicable in explaining online friendship formation.