Vocabulary plays a crucial role in language learning, especially in the context of second language (L2) acquisition. Understanding a word involves grasping its form, meaning, and usage, making vocabulary knowledge pivotal for both receptive and productive L2 skills. This study aims to assess vocabulary size, lexical density, and lexical diversity in argumentative essays written by Turkish English Language Teaching (ELT) students and explore their correlation with writing scores. The data, comprising 309 essays from 165 first-year and 144 fourth-year students at Anadolu University's ELT Department, were analyzed using the LFP to estimate students' productive vocabulary size, vocd-D for lexical diversity, and a lexical density formula. The results revealed that only lexical diversity had a significant correlation with first-year students' essay scores, explaining 7.8% of their performance. However, in the case of fourth-year essays, no significant effects of the variables were observed. Additionally, while the variables accounted for 8.7% of vocabulary scores in first-year essays, they did not significantly explain fourth-year essays' vocabulary scores. Therefore, this study suggests that while lexical features are important, they are not the sole determinants of writing scores.