This research aimed to explore the relation between syntactic awareness and writing composition in 129 Hong Kong Chinese children. These children were from a ten-year longitudinal project. At each year, a number of measures were administered. The 129 children's data of nonverbal reasoning at age 4, phonological awareness, morphological awareness, vocabulary knowledge at age 8, reading comprehension at age 12 and syntactic awareness and writing composition skills at ages 11 and 12 were included in this study. Syntactic awareness was longitudinally and uniquely predictive of Chinese children's writing composition, and children's performance in early writing composition was uniquely associated with their later syntactic skills, even when controlling for the contributions from age, nonverbal and verbal abilities, phonological awareness, and morphological awareness. The relationship between syntactic awareness and writing composition was mediated by children's performance in reading comprehension. These findings may suggest a reciprocal relation between syntactic awareness and writing composition, and this association may vary with ability in reading comprehension in Chinese children.