Engaging in argument from evidence is a fundamental science practice. However, preservice elementary teachers have difficulty constructing arguments. They need effective experience with argument construction in science. Written argumentation is crucial in the process of science learning. One of the approaches for learning science through argumentative writing is the science writing heuristic. However, only a few studies have investigated preservice science teachers' argument construction experiences with science writing heuristics. None of them involved preservice elementary teachers. Therefore, this study aimed to investigate preservice elementary teachers' understanding of argument components and the effect of the science writing heuristic on their quality of written arguments. A mixed-methods design was used in this study. The research was carried out for 10 weeks during the Science Laboratory Practices course. A total of 57 preservice teachers participated in nine laboratory experiments. They completed 409 laboratory reports structured based on the science writing heuristic. The scores were analyzed through a mixed effect model. Additionally, a questionnaire was used to investigate the participants' understanding of the argument components. The data were analyzed through content analysis. The analysis indicated that the science writing heuristic improved participants' written argument quality over time. It can be concluded that this approach is effective at improving the quality of written arguments. In addition, although most of their understanding of argument components is aligned with expectations, there are several shortcomings as well. This study contributes to the literature by facilitating the effective engagement of preservice elementary teachers in argumentation through science writing heuristics.