Debates about whether China’s diplomacy has become more assertive have been circulating in academics and policy realms since 2010. Xi’s presidency and the recent Wolf Warrior Diplomacy have raised more concern about the increasing assertiveness in China’s foreign diplomacy regarding its combative and hostile language. However, there has been relatively little effort to systemically examine the timing of the assertive rhetorical change in China’s diplomatic discourse and how assertive China’s diplomatic approach is. This thesis offers a measurable approach to studying China’s assertiveness using the Chinese Ministry of Foreign Affairs press conference transcripts between 2002 and 2022. I employ sentiment analysis and unsupervised topic modeling to provide a measurable approach and a large-scale empirical analysis of diplomats’ speeches. I show that the language of transcripts has become more negative and assertive starting in Xi’s second term of presidency (2018 onward). I also show that the increased assertiveness has been associated with what the Party considers “core interests” and Sino-U.S. relations.