Background: Studies have shown that word dictation and syntactic skills are significant predictors of written composition performance among Chinese children in elementary grades. However, there is a paucity of research on the bidirectional relationships between these two cognitive-linguistic skills (i.e., word dictation and syntactic skills) and Chinese writing skills. Methods: This paper reports the findings of a 1-year longitudinal study tracking the developmental patterns of Chinese writing among students in Grades 3 and 5 in Hong Kong. The participants were administered tasks involving cognitive-linguistic skills (working memory, word dictation and syntactic skills) and writing skills (narrative writing and expository writing). Results: Multiple regression analysis revealed that word dictation and syntactic skills in Grades 3 and 5 were significant longitudinal predictors of Chinese written composition performance in Grade 4 and Grade 6. Syntactic skills, but not word dictation, contributed a significant amount of unique variance to writing performance after controlling for the autoregressor effect of writing. The results also showed that written composition performance in Grades 3 and 5 contributed unique variance to individual differences in syntactic skills in Grades 4 and 6, after controlling for syntactic skills in the preceding year and other cognitive-linguistic variables. Conclusion: The patterns of results underscore the significance of syntactic skills in Chinese written composition in upper elementary grades and the bidirectional relationship between syntactic skills and written composition in Chinese.