Growth mindset (GM) theory conceptualizes that one's ability can be improved with effort (Dweck, Self-theories: Their role in motivation, personality, and development, Taylor & Francis, 1999) from a social-cognitive perspective. Self-determination theory (SDT) emphasizes the motivational source of learning, i.e. autonomous motivation (AM) and controlled motivation (CM). The two motivational theories can both lead to strategy use (SU) in writing contexts. To provide a more comprehensive understanding of motivation, this study investigated GM, AM, CM and SU through an integrated writing task. Participants were 465 secondary-four students in Hong Kong. Structural equation modelling showed that GM indirectly promoted writing performance through the mediation of AM and SU. Besides, neither motivations exerted direct effect on writing performance, whereas both did so via the mediation of SU. The findings unveiled the importance of GM in Hong Kong secondary school students' writing performance since it contributed to AM and SU. Regarding pedagogical implications, we encourage the proliferation of GM in every-day classroom.