This study aimed to investigate the effect of using two types of dictionaries (monolingual and bilingual) on Iranian intermediate EFL learners' vocabulary learning. An OPT (Oxford placement test, 2001) was administered among 90 students 60 of whom were selected as the participants of this study. They were sophomore students studying English as a foreign language, with the age range of 19-27. They were then divided into two comparison groups: Monolingual Dictionary (MD) and Bilingual Dictionary (BD) groups. A pretest of vocabulary was administered to both groups. Then both groups were given a five-session treatment. One group was taught vocabulary based on monolingual dictionary, and the other group was taught vocabulary through bilingual dictionary. After the treatment, the same version of vocabulary test was given to both groups as posttest to check the effectiveness of the treatments. The results of Paired-Samples and Independent Samples t-tests revealed that the effect of monolingual dictionary on learners' vocabulary learning was more than that of the bilingual dictionary use. The implications and recommendations will also be presented.