Teacher-level factors are theoretically linked to student outcomes in data-based instruction (DBI; Lembke et al., 2018). Professional development and ongoing support can increase teachers' knowledge, skills, and beliefs related to DBI, as well as their instructional fidelity (McMaster et al., 2020). However, less is known about how each of these teacher-level factors influences student progress during an intervention. The purpose of this study was to examine the association between several important teacher-level factors--teachers' writing instruction fidelity, knowledge and skills related to DBI, explicit writing orientation, and writing instruction self-efficacy--and students' writing growth. Participants were 49 U.S. elementary teachers and their 118 students struggling with early writing skills. Using hierarchical linear modeling, we found a significant positive relation between DBI knowledge and skills and student writing growth, but no relation was found between writing instruction fidelity, writing orientation, or self-efficacy and student writing growth. Implications for writing instruction fidelity measurement in DBI and professional development related to teachers' DBI knowledge and skills are discussed.