The study aimed to investigate the effectiveness and challenges of using monolingual dictionaries among second-year technical students at the Technical Institute of Research in Basrah. The research employed a quasi-experimental design and the required data was collected from 120 students who were categorized as upper intermediate learners based on their scores in the Oxford Placement Test. The study utilized a quantitative approach and gathered data using   a vocabulary proficiency test, a reading test, and a questionnaire. Also, a semi-structure interview was conducted on the use of the monolingual English dictionary, where participants were asked about their current skills, their perception of the importance of monolingual dictionaries, and their potential use of a monolingual English dictionary if they had good knowledge and skills. The results of the study showed factors such as limited familiarity with monolingual dictionaries, limited frequency use of such dictionaries as well as time-consuming searches and varied language proficiency levels are considered as the challenges in using monolingual dictionaries. Moreover, it was shown that the participants had a decent grasp of English vocabulary and using monolingual dictionary had a significant impact on Iraqi EFL learners’ reading comprehension. However, there are still areas where they may need improvement, particularly with less common or specialized words.  The study also found that participants highly valued the monolingual English dictionary for various reasons, such as clarifying word meanings, vocabulary expansion, language usage, language proficiency, self-study and autonomy, writing and communication, and as a reference for definitions. However, some participants expressed difficulties in understanding the dictionary's technical information and suggested that classes on monolingual dictionary skills could be beneficial. The study concluded   that monolingual English dictionaries play a crucial role in the language learning process, and providing classes on monolingual dictionary skills could further enhance students' language proficiency.