The aim of this study was to examine the argumentation skills of preservice science teachers on genetically modified foods during an argumentation process. Within case study research methodology, this study was carried out with 20 preservice science teachers identified through convenience sample from science education department. Knowledge Test for GMOs, Watson-Glaser Critical Thinking Appraisal and semi-structured interview questions were used to collect data. Findings showed that the groups with high level genetically modified food knowledge tended to use more qualified rebuttal and evidence. On the other hand, the groups with high level critical thinking skills were apt to exploit more qualified warrant, counter-argument, rebuttal and evidence. The research findings revealed that knowledge level and critical thinking skills are influential on argumentation skills. It was concluded that content knowledge is associated with self-efficacy and motivation in argumentation. However, critical thinking skills are associated with focusing, open-mindedness, understanding opposing ideas, and finding missing parts. In the light of this information, some suggestions was made for the development of knowledge and critical thinking skills for policy makers and educational scientists who have designed a teacher training program.