Purpose: This study examines and compares online incivility on China’s Weibo and the U.S.’s X (Twitter) amid the Russia-Ukraine conflict, aiming to unravel how different cultural and geopolitical contexts influence online incivility and identify factors that may influence the occurrence of online incivility in different national contexts. Design/methodology: This study collected and analyzed over 80,000 social media posts concerning the Russia-Ukraine conflict. By employing machine learning methods and moderation tests, this study compares online incivility in different country contexts. Findings: Twitter and Weibo show different level of online incivility across eight months in the discussion of Russia-Ukraine war. Conflict frame and negative sentiment both positively predict online incivility on Twitter and Weibo and these two factors both show higher prediction power on Twitter than on Weibo. Practical implication: This study highlights the necessity for platforms like X (Twitter) and Weibo to refine their moderation systems to address the predictors of online incivility, particularly negative sentiment and conflict framing. Social implication: This study provides evidence that cultural differences significantly impact online communication patterns and norms. It also finds that non-anonymous users might exhibit more uncivil behavior in politically charged discussions, seeking social approval. Originality/value: This research is one of the few studies to compare online incivility and its impact factors between China and the United States social media platforms. It shows how cultural differences influence the prevalence and predictors of online incivility and distinguishes the roles of negative sentiment and conflict framing in fostering incivility, with novel findings that challenge conventional beliefs about the impact of user anonymity on online discourse.