Vocabulary knowledge is considered one essential component of learning English. Although students are expected to know a certain number of words as stated in the Basic Education Core Curriculum B.E. 2551 (A.D. 2008), there is no study of English vocabulary size of Thai students. This study, therefore, explored the actual size of English vocabulary of Grade 10 students in small-sized schools in Khon Kaen. The objectives were (1) to identify vocabulary from English course texts of Grade 10 and (2) to measure the actual size of English vocabulary of Grade 10 students in small-sized schools in Khon Kaen. Instruments used to collect data included Vocabulary Size Test for Grade 10 Thai Students and interview questions. The tests were administered with 380 Grade 10 students from 19 small-sized schools in Khon Kaen. Six case students were interviewed for indebt data. The findings were (1) a list of Grade 10 vocabulary consists of 2,800 words to achieve the vocabulary expectation of Grade 10 (2,600-2,750 words), (2) the vocabulary size of the students was 849 words (31.74 percent of Grade 10 vocabulary expectation) and (3) all the six case students expressed that vocabulary knowledge did affect their English proficiency skills. Implications from the present study can be made for use in pedagogy and research.
EFL, Grade 10 Thai Students, Vocabulary Size, Vocabulary Size Test