Vocabulary ability is the basis of all language skills and closely related to learners' language proficiency in all areas. This study aims to measure vocabulary size, analyse the correlation between vocabulary ability and language proficiency. In this study, we selected forty Chinese students majoring in Korean language at Chinese universities as the subjects and randomly selected 100 words by frequency from a vocabulary list containing 10,641 words issued by the National Institute of Korean Language. We then calculated the learners' vocabulary size via the VST method. The study analysed the correlation between vocabulary scores and TOPIK scores. The development of learners' vocabulary ability and its effect on language proficiency were also examined. Vocabulary ability was proved to have a relatively large effect on language proficiency. However, vocabulary ability had a bigger effect on the language proficiency of the advanced learners group than on the basic learners group.