Based on the spiral of silence theory and social capital theory, this study suggested the process of SNS users’ public opinion expression on social issues. The motivational factors of willingness to speak out were presented as SNS bonding social capital, SNS bridging social capital, and perceived opinion support. SNS bonding social capital had a positive effect on perceived opinion support, and perceived opinion support positively affected willingness to speak out. However, there is no direct effect SNS bonding social capital to willingness to speak out. SNS bridging social capital did not affect perceived opinion support but directly affected willingness to speak out. This study empirically suggests that the process of expressing opinions varies depending on the type of SNS social capital. Besides, previous studies" limitations linking the existing social capital theory and the spiral of silence theory were identified. The process of public opinion formation was explored in consideration of today"s social media environment.