Abstract-Computer-Mediated Communication (CMC) has become an important part of human communication nowadays. When communicating using computers or digital media, people seem to behave differently when compared to face-to-face communication, especially in anonymous settings. When expressing their opinions in such an environment, people tend to write more directly and sometimes emotionally, without taking into consideration other people's face. Some may even be deliberately impolite or offensive on some occasions. Although impoliteness in CMC has started to attract researchers' attention, little is known in regard to factors triggering impoliteness or making people emotional in CMC. Drawing upon data collected from readers' comments on Japanese online news articles, this study has observed some clear differences in terms of impoliteness of the language used across threads of comments on different topics. Although on the surface, it seems that the topic of discussion has an influence upon the participants, this study claims that social identity, group face and gender are among the most important factors triggering impoliteness in Japanese CMC.