The purpose of this action research was to explore the effectiveness of a debate activity in prompting greater science teacher candidates' (TC) argumentation performance. We also investigated the separate impacts of the different aspects of the debate. Through comparing the quality of structural argumentation from 28 elementary TCs before and after the activity, we found that the debate has enhanced the TCs' argumentation skills in terms of the use of rebuttals. The qualitative data further revealed that the confrontational aspects of the debate, i.e. Clash and Extension, were most effective in arousing high quality dialogical argumentation as well as most risky in triggering negative attitudes towards argumentation. Finally, we suggested a revised rubric for argumentation assessment in which sophisticated argumentation skills are epitomized by not only rebutting but also synthesizing various information and multiple perspectives to reach a conclusion.