AbstractUsing monolingual dictionaries for educational purposes has become one of the major focuses of language learning in general and vocabulary learning proficiency in particular. Monolingual dictionaries cover so many areas for language learning such as grammar, spelling, and phonetics. Nonetheless, one of the most successful areas in which monolingual dictionaries can assist language learning is teaching and learning of vocabulary. Many studies have been conducted on different aspects of monolingual dictionary use in the Iranian context, but there have been few attempts to focus on its efficacy in the university environment for instructional purposes. This study aims to build on the previous studies in this area and investigate the effect of monolingual dictionary use on vocabulary learning proficiency. To do so, 113 undergraduate English Language and Literature students of Yazd University participated in this study. The subjects completed a fourteen-item questionnaire and a multiple-choice vocabulary test containing twenty questions. The researcher used Pearson product-moment correlation to analyse the strength of relationship between monolingual dictionary use and vocabulary learning proficiency. The results indicated that there was a strong positive correlation between the two variables (r = 0.607, r2 =0.36); furthermore, the large effect size of common variance shows that the more the use of monolingual dictionaries is reported, the more the proficiency of vocabulary learning is observed. The findings of the current research can have significant implications for improving the learners’ performance in learning vocabulary.