This study investigates the relationship between learner motivation and vocabulary size in English as foreign language (EFL) classrooms at Duhok University. The participants included 100 students (55 male, 45 female). All participants were pursuing their third years of study in an English department. Two instruments were employed: questionnaires regarding Motivation for Foreign Language Learning (MFLL) and Vocabulary Size Test (VST). The primary goal of this study is to determine which factors of motivation profoundly affect the foreign language learning processes of Iraqi EFL students and to what extent they should develop their depth and breadth English vocabularies in order to sufficiently acquire the language and elaborate the importance of both components in language acquisition. Findings showed that female students experienced both types of motivation, with the mean score of extrinsic motivation being 29.91, and that for intrinsic motivation being 31.20, while the mean score of male students was 27.10 for extrinsic motivation and 28.00 for intrinsic motivation. The VST ranged from 1,000 to 14,000 word-families, and the vocabulary size of both groups was over 6,000 word-families. Correlation analysis was conducted to determine the relationship between learner motivation and vocabulary size. For both groups, the results indicated no relationship between these two aspects of students' foreign language leaning. To investigate the difference between MFLL and VST, an independent samples t-test was utilized and no difference was found to exist between the vocabularies of both groups.