Biotechnology, as an emerging technology, has drawn much attention from the public and elicited hot debates in countries around the world and among various stakeholders. Due to the public's limited access to front-line scientific information and scientists, as well as the difficulty of processing complex scientific knowledge, the media have become one of the most important channels for the public to get news about scientific issues such as genetically modified organisms (GMOs). According to framing theory, how the media portray GMO issues may influence audiences’ perceptions of those issues. Moreover, different countries and societies have various GMO regulations, policies and public opinion, which also affect the way media cover GMO issues. Thus, it is necessary to investigate how GMO issues are covered in different media outlets across different countries. We conducted a comparative content analysis of media coverage of GMO issues in China, the US and the UK. One mainstream news portal in each of the three countries was chosen ( People's Daily for China, The New York Times for the US, and The Guardian for the UK). We collected coverage over eight years, from 2008 to 2015, which yielded 749 pieces of news in total. We examined the sentiments expressed and the generic frames used in coverage of GMO issues. We found that the factual, human interest, conflict and regulation frames were the most common frames used on the three portals, while the sentiments expressed under those frames varied across the media outlets, indicating differences in the state of GMO development, promotion and regulation among the three countries.