The purpose of this research was to examine the effects of implementation of argumentation in a physics classroom on students' reasoning. This research is both quantitative and qualitative in nature. Experimental design was used for the study. For the quantitative aspect of the research, students' prior knowledge in the beginning of the instruction and their knowledge after the instruction were measured and compared. For the qualitative part, students' reasoning was analyzed at the end of the instruction. The results of this study show that promoting argumentation in a classroom can enhance students' reasoning in science. Reasoning can lead to construction of scientific knowledge. Consequently, there can be significant gains in students' conceptual development by explicating, comparing and challenging ideas. (Contains 1 table.)