This study attempts to gain a deeper understanding of Thai EFL university learners’ vocabulary sizes and lexical collocations, both receptively and productively, and determine whether there is a significant relationship between them. Two hundred forty-two students majoring in English in the first and fourth years participated in this study. Four measures were used to assess the participants’ receptive and productive knowledge of vocabulary sizes and lexical collocations. The data were analyzed using descriptive and inferential statistics. The results showed that Thai university learners achieved significantly higher performance on receptive knowledge than productive knowledge, both in the tests of vocabulary sizes and lexical collocations. Besides, the fourth-year learners outperformed the first-year learners in all tests. The analysis of these findings revealed the degrees of learning in both vocabularies and collocations. Like vocabularies, receptive knowledge of collocational knowledge is easier to acquire than productive knowledge. The correlational analysis of the findings also revealed a positive relationship between participants’ vocabulary sizes and collocational knowledge. Together, the current results indicate that the gain in Thai university learners’ receptive and productive knowledge of lexical collocations fosters their vocabulary sizes and vice versa. Therefore, such findings highlight the roles of lexical collocations in fostering vocabulary learning and growth.