Scientific modelling plays a crucial role in students' science learning. Modelling proficiency and literacy development reinforce each other. This study investigates the relationship between teacher support of student literacy development and their modelling proficiency in the context of elementary project-based learning science environments. To explore the relationship, we sampled 557 students from 24 classrooms in 12 different schools. Data were analysed by multilevel mixed linear regression model analysis. Teaching strategies for teacher support of students' literacy development were identified based on our observation field notes. The findings suggest that teacher support of students' literacy development to engage them in the modelling practice is positively associated with their students' modelling proficiency. With teacher support of literacy development, elementary students are able to develop models that provide scientific explanations for phenomena related to core learning goals of the unit.