Using the Structure of Observed Learning Outcomes (SOLO) taxonomy as the analytic framework, this study examined the impact of the reasoning flow scaffold (RFS) on students' written arguments. Two classes with a total of 88 10th grade students in a school participated in this study. One class, set as the experimental group, was taught scientific argumentation with RFS whereas the control class received conventional argumentation teaching. They all experienced three argument assignments of writing scientific arguments and the measurement task before and after the teaching intervention. The results of data analysis showed that after teaching intervention, students in the experimental group performed significantly better than those in the control group on evidence and rebuttal while there were no significant differences on claim or reason between the two groups. Some implications and suggestions are provided in the last part of this paper.