The critical role of vocabulary knowledge in second/foreign language (L2/FL) learning has led the pedagogical researchers to seek efficient ways of presenting new words to L2/FL learners. Mimicking the organization of first language (L1) words in the mental lexicon, semantic and thematic clustering have been proposed by L2/FL acquisition scholars, with the former having its basis in the 'semantic field theory’ and the latter in the ‘frame semantics theory’. However, there is no general agreement on the effectiveness of these two types of word clustering in the acquisition of L2/FL vocabulary. Moreover, to the best of our knowledge, while these types of clustering have their roots in L1 cognitive studies, there is currently no research carried out investigating the similarities and differences between the bilinguals’ L1 and L2/FL lexical processing tendencies while presented with semantically and thematically clustered words. Therefore, this thesis sought to fill these gaps by firstly inspecting the effects of the semantic, thematic, and unrelated clustering types on Iranian Intermediate EFL learners’ vocabulary acquisition, and secondly, presenting an alternative approach toward word clustering research in L2/FL education by looking at the issue from a cognitive perspective. Using the Oxford Placement Test (OPT), 45 intermediate English learners were selected to participate in this study. To measure their English vocabulary knowledge, the Vocabulary Size Test (VST) was also administered to them. To accomplish the first aim of the study, English vocabulary was instructed in three conditions of semantic, thematic, and unrelated and then the participants’ word learning was tested employing an L2-to-L1 translation test. To achieve the second aim, the participants were presented with Persian and English prime-target word pairs in the mentioned three clusters by employing masked priming lexical decision tasks (LDTs). The vocabulary acquisition was analyzed using a general