Despite the well-established role of lexis in proficiency in the four skills, a great deal of research has focused on the importance of vocabulary breadth and depth in reading. Therefore, the neglect of productive skills has motivated recent calls to inspect the impact of lexical dimensions on writing ability. Using Partial Least Squares Structural Equation Modelling (PLS SEM), this study examines the contributions of vocabulary size and depth aspects to EFL learners' narrative writing ability. For this purpose, the researchers administered the Updated Vocabulary Levels Test (the UVLT), the Productive Vocabulary Levels Test (the PVLT), the Word Associates Test (the WAT), and a narrative writing task to 77 EFL undergraduate students. The participants' scripts were scored, following IELTS analytical band descriptors. Results of the PLS SEM disclosed that vocabulary size and depth explained around 45% of the variance in the writing scores of the subjects. They also revealed that, among the variables examined, receptive vocabulary size was the only statistically significant factor that contributed the most to writing ability. The implications for writing assessment and instruction are discussed.