The aim of the present study was to investigate the relationships of Iranian EFL Learner’s Crystallized and Fluid Intelligences with their Vocabulary Size. The Participants of the study were 100 male and female learners from Islamic Azad University of Mashhad in TEFL course that are ranged from 20 to 40 years old. To administer the study, an experimental and quantitative analysis was conducted and the relationship between variables was measured using three instruments namely, Baddeley’s (1968) Grammatical reasoning Test for Fluid intelligence, Persian C-Test (2015) for crystalized intelligence, and Nation’s (2012) vocabulary size Test. Data were analyzed using SPSS version 21. First, correlational analyses were conducted to find out correlation coefficients between the variables. Then, multiple coefficient tests were done to find Beta weight of variables. The results obtained from correlational analyses showed that there is a significant relationship between crystallized intelligence and vocabulary size, while there is no significant relationship between learners’ fluid intelligence and their vocabulary size. Besides, regression analyses showed that the model consisting of both crystallized and fluid intelligences explains a small but significant portion (8%) of the variance in vocabulary size. It was also concluded that fluid intelligence does not significantly predict vocabulary size, but crystallized intelligence significantly predicts vocabulary size. The study implies that crystalized intelligence should be considered in language learners’ curriculum to achieve the teaching and learning purposes. تفاصيل المقالة