The previous researches about knowledge sharing were proceeded in terms of KMS (Knowledge Management System) in center. However, knowledge sharing is recently applied to Internet space, which is open to every users, as well as KMS, which is qualified for restricted people. For example, some portal sites, such as Naver, the most popular portal in Korea, have virtual spaces to share users' knowledges and it is common that many users use the spaces. Knowledge sharing online, compared with KMS, will be more advanced to promote intention for knowledge sharing because of the character of Internet space that is open to all users. Nevertheless, there are few researches about knowledge sharing in the Internet. Considering this situation, this study is attempted to figure out the factors to promote Internet knowledge sharing, based on social capital theory and self-expression concept. A survey of experienced Internet user and PLS (Partial Least Square) were utilized for analysis. The test of this study reveals that social capital and self-expression are significant factors to influence knowledge sharing intention, and that also personal innovation and self-efficacy are significantly related to the self-expression. However, personal innovation does not have significant impact on social capital. According to the result, self-expression, as well as trust and system itself, has significantly effect on knowledge sharing intention in order to promote knowledge sharing in the Internet.