The purpose of this study was to examine the effects of argument structure on students' writing in implementation of argument-based inquiry using the Science Writing Heuristic (SWH) approach. Participants of this study were 108 8th grade students (three classes). Two classes (68 students) were assigned to an experimental group, and the other class (35 students) was assigned to a comparative group. The experimental group was taught argumentbased inquiry using the Science Writing Heuristic (SWH) approach, while the comparative group was taught with the traditional teaching strategy. After implementing this program, the two groups were asked to write summaries using structured argumentation in their writing. The result of this study showed that the experimental group used better argument structure and multimodal representation such as pictures, graphs and examples in evidence than the comparative group. The quality of evidence used in the students' writing was different between two groups. Students of the comparative group only listed fragments of science concepts for evidence to support their claims, but students of the experimental group explained science concepts by giving specific examples. The findings show that argumentbased inquiry using the SWH approach was effective on argument structure in students' writing.