With the development of Internet and transition to information society, social capital is expanding to online from the traditional offline context. Especially with the widespread of social network service(SNS) the number of SNS users is increasing sharply and the importance of online social capital has been more and more significant. Most studies on social capital focused on organizational aspects but few studies have payed attention to personal aspect. Empirical studies on the relation between SNS and social capital were seldom conducted in previous studies. Based on the theory of social capital this study targets on the relationship formed through SNS and analyzes on how the relationship affects the perceived social capital. In this study ``self-presentation``, ``playfulness`` and ``critical mass`` are posited as the antecedent factors of ``SNS usage``. This study proposes a research model to examine the effect of ``SNS usage`` on ``relationship reinforcement``, ``relationship building`` and ``perceived social capital``. According to the results of empirical analysis, ``self-presentation``, ``playfulness`` and ``critical mass`` can generate significant positive influence on ``SNS usage``. It also confirms not only the effect of ``relationship reinforcement`` and ``relationship building`` formed through SNS on ``perceived social capital`` but also relationship between the social capital formation and SNS usage. The outcome obtained in this study can be applied in developing SNS services.