Media self-censorship refers to non externally compelled acts committed by media organizations aiming to avoid offending power holders such as the government, advertisers, and major business corporations. While self-censorship constitutes a major threat to press freedom in Hong Kong under China, recent studies have shown that Hong Kong journalists have maintained a strong sense of professionalism. The coexistence of professionalism and self-censorship poses important challenges to news organizations: How is self-censorship effected as professionalism does not favor its practice? How can news organizations minimize the conflicts between self-censorship and professionalism so that news operations will remain stable, smooth, and efficient? Drawing on the literature on newsroom social control, we tackle the above questions by focusing on the internal structure of and the interactive dynamics within newsrooms. Methodologically, this study draws on both representative survey and in-depth interview data. It is found that self-censorship is effected through selective positioning and assignment, observational learning of tacit rules, the giving of ambiguous orders, and the use of professional or technical reasons to justify questionable news decisions. Meanwhile, some journalists also developed their own operational tactics to resist what they perceived as self-censorship attempts. The theoretical implications of the findings are discussed.