In reading news stories online, people are exposed to others` comments to a news story. In spite of popularity of comments to a news story online, the understanding of why and who posts a comment is still lacking. To complement scarcity and have a better understanding of comment-posting online, this study examined psychological factors which affect the likelihood of posting a comment to a news story online. In particular, three variables were considered: The first variable was communication efficacy, that is, an individual`s belief about communication practices and systems, under the supposition that the people who have greater levels of communication efficacy are more likely to post a comment. The second variable was perception of public opinion, to test that when and if people judge their position of the issue as favored by the majority, they tend to post a comment. And finally, the tone of existing comments was included, to test if the tone of comments affects the willingness to post a comment. The results showed that firstly, people at a high level of communication efficacy are more likely to post a comment compared to people at the low level of communication efficacy. Secondly, the perception of public opinion partly influenced the willingness to post a comment. Especially, when people believe communication systems contribute to develop society and also when judged that their opinion is the majority`s opinion, they are more likely to post a comment. Finally, the tone of the comments influenced the willingness to post a comment only on the condition that people are confident of the communication practice and are exposed to emotional comments.