The specific size of vocabulary has been prescribed in Thailand's basic education curriculum for decades, but less is known about Thai students' vocabulary size. This study focused on Thai freshmen who just finished their 12 years of basic education. The objective of the study was to assess their vocabulary size and examine the relationship between vocabulary size and years of study. The participants were 484 Thai freshmen from four public universities and three private universities across Thailand. The results showed that Thai freshmen possessed around 4,200 word families. The Pearson correlation coefficient between vocabulary size and years of study was significant and positive yet relatively weak (r=0.201, p<0.01) which indicated that years of study were partly related to vocabulary size.