Research has shown that the use of noun phrases is a salient characteristic favored in academic writing (Biber et al., 2011; Biber & Gray, 2011; Parkinson & Musgrave, 2014). Although studies have examined the structure of noun phrases, further research is needed to deepen our understanding of noun phrase complexity in order to inform the instructional content of academic writing courses. In response to this need and the call for more empirical evidence for the developmental stages for complexity features proposed by Biber et al. (2011), this study aimed to investigate differences in the use of noun modifiers among second language learners of English at varying proficiency levels. A corpus consisting of 286 essays, written by international students in response to an English integrated writing placement test, was analyzed for grammatical structures of noun phrases. Results indicated that more proficient writers employed significantly more attributive adjectives, prepositional phrases, finite relative clauses with non-animate head nouns, and nonfinite relative clauses. These findings support previous research on the relationship between writing proficiency and the use of noun modifiers and provide implications for both academic writing instruction and the development of writing assessment criteria.