Scholars often blame the occurrence of aggressive behavior in online discussions on the anonymity of the Internet; however, even on today’s less anonymous platforms, such as social networking sites, users write plenty of aggressive comments, which can elicit a whole wave of negative remarks. Drawing on the social identity and deindividuation effects (SIDE) model, this research conducts a laboratory experiment with a 2 (anonymity vs. no anonymity) × 2 (aggressive norm vs. non-aggressive norm) between-subjects design in order to disentangle the effects of anonymity, social group norms, and their interactions on aggressive language use in online comments. Results reveal that participants used more aggressive expressions in their comments when peer comments on a blog included aggressive wording (i.e., the social group norm was aggressive). Anonymity had no direct effect; however, we found a tendency that users’ conformity to an aggressive social norm of commenting is stronger in an anonymous environment.