Vocabulary Size Test (VST) is a test to measure learners’ receptive vocabulary size by Paul Nation. Beside English-English version, VST has other five bilingual versions. This article is to describe the validation of its Mandarin version, i.e., English-Chinese version. Data shows that the bilingual English-Chinese version VST’s overall difficulty index, discriminating power, and internal consistency are all statistically satisfactory. One-way ANOVA test and Post Hoc test evidenced that the English-Chinese version VST can differentiate learners from different levels with high validity.