The aim of this study is to investigate the relationship of preservice science teachers’ risk perceptions related to a socio-scientific issue with decision making mechanisms. For this purpose, holistic multiple case study design among the qualitative research methods has been used. This study has been conducted with 18 preservice teachers selected by purposeful sampling method from a state university which is located at Central Anatolia Region. As data collection tools, Risk Perception Scale Related to Genetically Modified Organisms (RPS-GMOs), Chocolate Choosing scenario, and interview questions have been used. According to the research findings, preservice teachers have presented three types of decision making mechanism model. In the Type 1 decision making mechanism, they reduce the alternatives by directly eliminating the alternatives which are not appropriate for themselves and make comparison among the remained alternatives in terms of advantage-disadvantage. In Type 2 decision making mechanism, preservice teachers make a selection by directly weighting a criterion. After the selection, preservice teachers make advantage-disadvantage comparison between the selected alternative and the other alternatives. In Type 3 decision making mechanism, preservice teachers analyse each alternative according to its advantages-disadvantages and, finally, they make a decision by weighting a criterion. In the study, it is identified that the ones who have high risk, use Type 1 decision making mechanism, the ones who have medium risk, use Type 3 decision making mechanism and the ones who have low risk, use Type 2 decision making mechanism. It is suggested to provide decision making skills enhancing education to preservice teachers, and to hold the risk on the medium level on the controversial and risky issues.