This study examines the relationship between developing writer's vocabulary breadth, vocabulary depth, and writing abilities. Participants were EFL Chinese junior-high students, of whom there were 120 Grade 8 and 147 Grade 9 students. Measures included tests of vocabulary size, word association, and morphological awareness. Students' writing samples were coded with a five-point rubric covering six categories: focus and idea generation, organization, word choice, grammar, sentence fluency, and length of writing samples. Standard multiple regression analyses showed that vocabulary breadth made a bigger contribution to writing performance among the EFL 8th and 9th graders, whether the outcome variable was the overall writing quality or individual groups of scoring components. In addition, some aspects of vocabulary depth such as adjective synonyms were only related to the 9th graders' writing performance, suggesting that depth of vocabulary may exert a growing impact on EFL learners' writing development. These findings support the need for systematic vocabulary instruction in EFL classrooms, as it can optimize the strong relationship between vocabulary breadth and depth and their impact on writing development.