This study aimed to determine the effect of the model-based science writing heuristic (M-SWH) approach on students' conceptual understanding and construction of model-based argumentations in elementary school science. Participants (N = 107) were students enrolled in 4 fourth-grade classes of a public elementary school. Two classes were assigned as the treatment group, and the other two classes were assigned as the comparison group in this quasi-experiment study. Students in the treatment group were instructed using the M-SWH approach, while a traditional approach was used in the comparison group. Tests measuring students' conceptual understanding of "properties of matter" and "lighting and sound technologies" units were administered to both groups as pretests and posttests; the M-SWH reports were used to assess the treatment group's ability to construct model-based argumentation. Descriptive statistics, t-tests, correlation analysis, and repeated measures ANOVA were used for data analysis. The results demonstrated a statistically significant difference in concept tests between the two groups in favor of the treatment group. These students' ability to construct arguments using models developed significantly throughout the implementation of the M-SWH approach. Moreover, treatment group students' conceptual understanding improved with their quality of writing.