The purposes of this study are to understand the effects of using Synchronous Discussion and Reflection System (SDRS) platform in the elementary students' perception of the learning environment and students' cognitive outcomes regarding the Genetically Modified Food (GMF). A total of 22 students from Elementary School in Taichung Taiwan joined this study, consisted of 10 students in the experimental group and 12 students in the control group. The results of the study showed: (1) The experimental group using SDRS have advantages in self-directed discussion and reflection than the control group by teacher guidance; (2) students' perception in the KB-Based learning environment from the experimental group is as good as the control group; (3) learning outcomes in the experimental group are better than the control group. So this study concludes that the Knowledge Building-Based Learning using SDRS as a platform for self-directed discussion and reflection was equally more effective than with using the discussion guide by the teacher in class. This study also suggests that elementary school students need more time to adapt using the platform in KB-based learning to get better result both in their perception of the learning environment and their learning outcomes.