The present study investigated the association between vocabulary size and depth of Iranian EFL learners at different language proficiency levels. Additionally, the extent that this relationship could be different for low- and high-word-frequency bands was probed. In so doing, the Word Associates Test (WAT), the Vocabulary Levels Test (VLT), the monolingual Vocabulary Size Test (VST) and its bilingual Persian version were administered to 122 Iranian English as a Foreign Language (EFL) learners (F=79, M=43) who had been classified into three language proficiency levels by means of administering the Oxford Quick Placement Test (OQPT). The findings indicated the following: (a) vocabulary size and depth had a significant correlation for the lower-intermediate students; (b) size and depth dimensions had a moderate association for the upper-intermediate participants; (c) vocabulary size and depth were not significantly correlated for the advanced EFL learners; (d) the relationship between the higher-frequency bands of vocabulary size and vocabulary depth was significant for lower proficiency levels; and (e) the lower-frequency vocabulary size was not correlated with vocabulary depth for any of the proficiency levels. The results have implications for vocabulary depth instruction for higher language proficiency levels and suggest teachers, test developers, and materials designers to incorporate the dimension of word associations into the construct of word knowledge.