The aim of this study is to investigate the effect of making argumentation practices on the conceptual understanding level and decision making styles of pre-service science teachers on socio-scientific subject, genetically modified organisms (GMO). The sample of this study consists of 48 Pre-service science teachers studying in the 2nd grade of the science education department, Faculty of Education, in the 2018-1019 academic year. 24 of the pre-service science teachers are the experimental group and 24 of them are the control group. Case study was used in the study. In the experimental group; within the scope of 5E teaching model; with materials and animations containing Toulmin's argumentation model; the control group was processed with traditional teaching methods. Conceptual understanding levels of Pre-service science teachers, who constitute the experimental and control groups, about GMO before and after the teaching activities, were measured. As a data collection tool in the research; the open-ended question and Likert type scale consisting of 12 items, which are based on GMO, based on expert opinions and have scope validity, is the Knowledge Test on Genetically Modified Products of University Students (KTGMOUS), pre-test and post-test. It was applied as a test. In addition, a five-point Likert-type "Decision-Making Styles Scale" consisting of 24 items was applied to the experimental group in the form of pre-test and posttest. Findings obtained at the end of the study are presented.