This study aimed: 1) to construct an English vocabulary size test for Mathayomsuksa 4 students and 2) to establish the validity and reliability of the test. Six steps adopted in constructing the test were; 1) setting the test specifications, 2) establishing target words, 3) writing the first draft, 4) seeking for the test validity, 5) piloting for the test reliability and 6) adjusting the test for use. The validity was from the results of the Index of Item-Objective Congruence (IOC) of three experts. The reliability was calculated using Kuder-Richarson Formula 20 (KR20). The Vocabulary Size Test for Grade 10 Thai Student (G10VST) is a multiple choice format with five options. It consists of 70 items. The validity and reliability of G10VST was 0.96 and 0.71 respectively. The G10VST can be used in English vocabulary assessments in pedagogy and research.