In contrast to offline-based interpersonal relationships, the development of online-based interpersonal relationships have been somewhat controversial, and researchers still cannot clearly define online-based interpersonal relationships according to any developmental level. For this reason, this research aimed to investigate the developmental process of SNS users’ online-based interpersonal relationships based on the social penetration theory. From October 12, 2012 to January 21, 2013, in-depth interviews were conducted on 10 voluntary research participants who were active in online-based interpersonal relationships with 30 online friends. Phenomenological qualitative research derived from transcribing the interview results in verbatim recording showed that online-based interpersonal relationships could develop in a different manner depending on whether personal information was open or closed in terms of cyber space’s environmental characteristics. After developing interpersonal relationships from the orientation stage during a brief period, self-disclosure SNS users showed a tendency to stay in the exploratory affective exchange stage during a long period. And those who had the experience of new offline-based meetings in the exploratory affective exchange stage revealed a tendency to move onto the full affective exchange stage. Self-closure SNS users showed a similar tendency to develop from the orientation stage through the exploratory affective exchange stage to the full affective exchange stage compared to offline-based relationships. Depending on whether they had a sense of closeness or not, they stayed at the full affective exchange stage or at the stable exchange stage. In general, very few people of self-closure SNS users reached to the stable exchange stage. During the disintegration process of online-based interpersonal relationships, the research participants reported that they were not able to continue relationships due to reasons such as reduced favors towards other sides, misunderstandings, and conflicts regardless of the specific developmental levels. Therefore, although online-based interpersonal relationships showed a similar developmental level to offline-based interpersonal relationships in terms of the developmental, intensified, and disintegrated processes, the results implied that online-based interpersonal relationships developed differently, depending on whether personal information could be open or closed in terms of cyberspace’s environmental characteristics.