Background: Writing is a particularly demanding activity, which poses unique motivational challenges for students. Despite the wealth of research on the relation between writing motivation and writing performance, little is known about the role of students' writing frequency in writing motivation and writing performance. Aims: We aimed to: (1) examine structural relations among two motivational variables (i.e., self-efficacy and attitudes), a behavioural variable (i.e., writing frequency), and writing performance; and (2) inspect whether these relations varied across two text genres (i.e., narrative and opinion texts) and across two educational levels (i.e., students in grades 5-6 and grades 7-8). Sample: Six hundred and five students from grades 5-8 participated in this study. Methods: Students completed self-report scales and wrote narrative and opinion texts. We conducted multiple-group structural equation modeling to analyse the data. Results: Regarding narrative texts, digital writing frequency was significantly associated with text quality for students in grades 7-8, but this relation was not significant in students from grades 5-6. Both attitudes and self-efficacy for self-regulation made a direct contribution to narrative text quality across educational levels. In addition, attitudes were associated with both literary and digital writing frequency across educational levels. Concerning opinion texts, no significant differences emerged in terms of educational level. Attitudes contributed to both literary and digital writing frequency as well as to opinion text quality across educational levels. Conclusions: This study underlines the fundamental contribution of motivational variables to students' writing performance. Accordingly, teachers need to adopt motivation-enhancing practices in writing instruction across grade levels.