This study reports the effect of computer models as formative assessment on high school students' understanding of the nature of models. Nine high school teachers integrated computer models and associated formative assessments into their yearlong high school chemistry course. A pre-test and post-test of students' understanding of the nature of models using a published measurement instrument on the nature of models were conducted. A four-step hierarchical multiple regression and a two-level (level 1--student and level 2--teacher) hierarchical linear modeling were used to test the effect of the intervention on students' understanding of the nature of models. Our analysis revealed a significant effect of frequencies of using computer models for four of the five sub-scales related to the nature of models. The implications of these findings are that, as students have more experience using computer models in their classroom, they develop a better understanding of the nature of models. However, their understanding of models as multiple representations did not show a significant improvement, possibly due to the lack of support from teachers, who in turn need both content and pedagogical supports within their teaching.