This study examined the contribution of the constructs of orthographic processing (orthographic choice and orthographic choice in context), syntactic processing (grammaticality and sentence integrity), and verbal working memory (two reading span indicators) to written Chinese composition (narration, explanation, and argumentation) in 129 15-year-old L2 learners. A matrix task was also administered as a control task to tap cognitive flexibility. Structural equation modeling (SEM) analysis with written composition as a latent variable revealed orthographic processing and working memory as two significant, independent contributors, whereas the unique contribution of syntactic processing was not significant. Subsequent SEM analysis with narration, explanation, and argumentation as separate endogenous variables found varied patterns of the contribution of each latent predictor to written composition in different genres. These patterns are discussed in light of the importance of attention to learners' developmental stage and genre-sensitive measures to capture the psycholinguistic and cognitive underpinnings of written composition in L2 Chinese.