This study aims to explore what factors influence Internet users' willingness to share knowledge on social network sites. With the success and popularity of social network sites such as Google +, Facebook, Line, or Zalo, people realize that collective collaboration has become the cornerstone of the website's success. More and more Internet users are using social network sites for knowledge-sharing, which is playing a vital role in developing collective and collaborative knowledge. The research model in this study is based on Reasoned Action Theory (TRA) with a sample of 310 Vietnamese Internet users. A path analysis was used to test all the hypotheses using AMOS. We found that Absorptive Capacity and Organizational Self-Esteem have a positive and significant influence on Knowledge-Sharing Attitude. However, there is no positive and significant influence of Expected Return on Knowledge-Sharing Attitude. Besides, Knowledge-Sharing Attitude expectedly has a positive and significant influence on Knowledge-Sharing Intention. It is worth noting that although Expected Return has no positive and significant influence on Knowledge-Sharing Attitude, this finding helps to prove that in such an open online environment, Expected Return becomes an unimportant predictor for Knowledge-Sharing Behavior among Internet users.