This study discusses Japanese computer-mediated communication (CMC), specifically focusing on messages sent to open-access bulletin board system (BBS) websites. It first compares quantitatively CMC language with speech and writing and finds that interjections distinguish speech from CMC and writing. CMC is distinguished from writing by uses of particles. Uses of auxiliary verbs separate the two target websites, Channel 2 and Yahoo within CMC. Based on the linguistic characterisation of the CMC language, the thesis further discusses qualitatively politeness and impoliteness behaviours in the two target BBS websites with contrasting linguistic features. This study points out that one theory of politeness proposed by Brown and Levinson (1987) can explain impolite behaviour in Channel 2, but not politeness in Yahoo, and that another theory proposed by Ide (1989) can explain polite behaviour in Yahoo, but not impoliteness in Channel 2. A third theory from a discursive approach proposed by Locher and Watts (2005) is shown to be capable of synthesising the two contrasting situations. In the following chapters, concepts of online community are employed to discuss politeness issues in conjunction with the sense of community. Here differences in discussion topics are found to be relevant to both the sense of community and linguistic choices of polite/impolite language, across the two target websites. Seemingly impolite behaviour in Channel 2, where users have a strong sense of community can be explained by the concept of contextual appropriateness by Watts. Underlying the topics specifically addressed in the thesis, this study also identifies the greater role played by technology in Japanese CMC than in English CMC. It fills a research gap in linguistic study on CMC language in Japanese, politeness and impoliteness research in online context as well as online community studies in Japanese cultural contexts. It is expected to contribute to understanding Japanese CMC linguistically and socio-culturally, as well as politeness and impoliteness and online community research in general.