This paper reports an attempt to develop and validate a bilingual Persian version of the Vocabulary Size Test (VST). Due to the particular educational system in Iran, there is a dire need for a test that can effectively estimate English learners' vocabulary sizes. Previous research (Nguyen and Nation, 2011) has indicated that bilingual versions of the VST can be more efficient than the monolingual one. A calibration of the Persian version of the test with 190 English learners indicated that the test enjoys a high level of validity and reliability. The results of a factor analysis revealed a single construct, presumably word knowledge, is underlying the test. A one-way between-subjects ANOVA also indicated that the test can effectively distinguish between different proficiency levels. The hypothesized difficulty order was also realized in the data though it was found that clusters of 1,000 word levels provide more meaningful difficulty levels as they are less susceptible to the idiosyncrasies at each 1,000 level. The results were also against the common assumption in the literature that not all test takers should sit the entire test. The administration of the whole test leads to a more valid estimate of the examinees' vocabulary sizes. (Contains 1 note, 9 tables, and 3 figures.)