One of the productive word-formation processes in English and Persian that involve category change is conversion. The present study aims to compare and contrast conversion in English and Persian in terms of its productivity, the word classes that undergo this process, as well as its directionality. The research method is descriptive-analytical in order to carry out a contrastive analysis of conversion in English and Persian. To this end, various cases of conversion are investigated in both languages separately to identify the most frequent types of conversion in either language with regard to the parts of speech that are the input and output of the conversion process. The samples are taken from monolingual dictionaries, printed material, and online journalistic texts. Based on the results, it can be concluded that while conversion is a productive word-formation process in Persian, it acts differently compared with the English language in variety, frequency, and its target of application. Modern English has only word-to-word conversion; however, in Modern Persian, there are three types of conversion: word-to-word conversion, stem-to-word conversion, and word-to-stem conversion. Regarding rank-based conversion, in both English and Persian phrase-to-word and sentence-to-word conversions were found. In both languages, the class-based conversion process is quite productive but differences can be observed in the input and output of the conversion process. پرونده مقاله