The aim of this research was to investigate the effects of science teaching activities supported by discussion and decision-making processes on critical thinking dispositions of teacher candidates. In the study, quasi-experimental pre-test and post-test control group design were used. The working group of research was composed of 101 teacher candidates, 49 of whom were in experimental group and 52 of whom were in control group. In the study, "California Critical Thinking Disposition Inventory" was administered as pre-test and posttest to the experimental and the control groups. The teacher candidates in the control group participated in largegroup discussions, while in experimental group were conducted discussion and decision-making processes in small groups. According to the findings obtained from the study, statistically significant increases were revealed in the critical thinking dispositions both of the groups. However, mean scores of control and experimental groups were compared, there was aa meaningful difference in favor of the experimental group.