Lexical richness makes an important contribution to L2 writing quality. To explore its importance, the study aims to identify and explain how lexical richness manifests in argumentative essays written in the final exam of reading and writing class by thirty-five undergraduates. The lexical richness consists of four interrelated elements: lexical diversity, density, sophistication, and fluency. Detailed text analysis can identify these elements in EFL students' academic writing. The correlation analysis showed that the use of lexical diversity, sophistication, and fluency all affect writing quality and can be seen differently in a text depending on different score ranges, vocabulary knowledge and linguistic performance. Further, the regression analysis revealed that the lexical sophistication was found to be the most significant predictor that contributes to writing quality. In sum, the lexical richness displayed in written text is a result of a person's underlying vocabulary knowledge. This study ends with a pedagogical implication for teaching lexical richness in EFL academic coursework.