The main objective was to explore the reliability and validity of the bilingual English-Thai version of the vocabulary size test adapted based on Nation and Beglar (2007). The findings indicate that (1) the test is reliable; it produced consistent and stable results, (2) the test is valid; it well measures subjects’ vocabulary size, (3) the words in each level are quite organized; the scores generally decline from the beginning level which is the easiest one (1st 1000 word level) to the highest level which is the most difficult one (14th 1000 word level), and (4) the test result shows that only 14 word levels are not sufficient to access the subjects’ total vocabulary size; the test should contain more word levels. In all, the bilingual English-Thai version of the vocabulary size test is a reliable and valid measure of Thai students’ vocabulary size.