Abstract: This article investigates the different effects of vocabulary size and those of depth of vocabulary knowledge on reading proficiency. It first reviews some relevant theories and studies in this field. Then the author carries out an empirical study to investigate the vocabulary size and vocabulary depth of 146 second-year English-major students. The scores of these two tests are compared with the score of the “Reading Comprehension” section from TEM-4. The results of the study reveal that both vocabulary size and vocabulary depth are highly and positively correlated with reading proficiency, but vocabulary size is more conducive to reading proficiency.