The present study, a quantitative lexical analysis, examines the extent to which vocabulary size and lexical diversity contribute to writing scores on advanced non-native speakers’ and native speakers’ academic compositions. The data consists of essays composed by 104 adult non-native English learners enrolled in advanced second language writing courses and 68 native speaking university students in a first-year composition course. The lexical diversity of the sample essays is quantified by both the Measure of Textual Lexical Diversity (MTLD) and the voc-D while vocabulary size is measured by CELEX word frequency means, three instruments that are available in the computational linguistics program Coh-Metrix 3.0. Writing scores are provided by three independent raters’ evaluations according to the TOEFL iBT Independent Writing Rubric. Results from a binary logistic regression reveal that lexical diversity has a significantly greater impact on writing score than vocabulary size (p < .01). Nevertheless, a series of MANOVAs indicate that vocabulary size initially facilitates writing scores at the lower proficiency levels, but it is an essay’s lexical diversity that promotes it to the higher score levels. Additional findings from the MANOVAs demonstrate that native speakers’ profiles of lexical diversity and size are significantly different from their non-native peers (p < .001). The lexical profiles also differed significantly among the individual score levels of the TOEFL iBT rubric (p < .05). A final outcome from a Pearson’s product moment correlation analysis shows that iii vocabulary size has only a moderate relationship to lexical diversity, suggesting that variation of mid-range vocabulary may be more important to writing proficiency than the use of more sophisticated terms that occur less frequently in natural language. Implications for practice suggest that it is not enough to simply teach vocabulary words in the L2 composition classroom, but also to guide learners in how to employ these words in a varied manner within their writing. Furthermore, the results of this study indicate that teachers should spend more time on helping students use medium frequency words along with synonyms of a similar frequency rather than teaching students infrequent vocabulary, which may appear to sound more advanced.