As an intimidating school subject for students globally, writing requires a high level of motivation, especially for second language (L2) learners. Despite abundant research on motivation in both first language (L1) writing and L2 learning, L2 writing motivation warrants more attention. To bridge the gap, the study intends to present an overall picture of Hong Kong primary students' English writing motivation level and the sophisticated interplay between two important motivational variables (i.e. self-efficacy and task value). Four hundred and thirty-three 4th--6th graders participated in the study. The findings revealed a medium level of writing motivation for the students. MANOVA results found significant differences in the levels of self-efficacy and task value (i.e. interest, and importance and usefulness) across the grades. The 5th graders' motivation level was significantly lower than the 4th and the 6th graders'. Additionally, Structural Equation Modelling (SEM) suggested an intricate relationship between task value and self-efficacy. Specifically, interest predicted all the aspects of self-efficacy in English writing (i.e. self-efficacy in language, organisation, content, grammar, and process), whereas importance and usefulness predicted only self-efficacy in grammar and process. Important implications are discussed.