The purpose of this study is to explore the effects of lexical measures on Chinese EFL students' English writing proficiency. To achieve this goal, a corpus of English argumentative essays written by Chinese EFL learners which contained writing scores by an automatic grading computer system was constructed. For this study, various lexical indices were generated through computer tools, then regression analyses were performed to examine the effects of these measures on Chinese EFL students' essay scores. Specifically, regression analyses with the lexical indices as predictor variables and the student essay score as the response variable were conducted. The results showed that a total of ten lexical measures had significant effects on the students' essay scores. The findings not only confirm results from previous studies but also have important pedagogic implications for EFL classroom education.