Vocabulary plays an important role in language learning． This paper aims to study the predictability of the Vocabulary Size Test ( VST) on language proficiency by looking into the correlation between the vocabulary of non-English majors and their scores of College English Tests ( CET-4 and CET-6) ． The study shows that vocabulary size is significantly related to CET-4 but not associated much with CET-6． It may be a warning for the non-English majors and their teachers，who focus much on breadth and reception of vocabulary knowledge while attaching little importance to depth and production of vocabulary knowledge．