Introduction:It is believed that different components of our linguistic capabilities are not impaired to the same extent in aphasic patients. Moreover theoretical issues on aphasia can be researched on patients with different languages. Thus, we aimed to study the dissociation of inflectional and derivational morphology by assessing the performance of 8 Persian bilingual aphasic patients in producing Persian derivational and inflectional words. Materials and Methods:To explore the capability of patients in using derivational and inflectional words, in addition to obtaining a brief history from all patients and an image of the impaired regions of the brain, two types of tasks, word-repetition and spontaneous speech tasks, were administered to them. The results were then statistically analyzed to see which part of their word-formation competence, derivational or inflectional processes, was impaired more seriously. Results:The results of word-repetition and spontaneous speech tests indicated that patients did better at derivational morphology. In addition, a quantitative analysis revealed a gap between scores for derivational and inflectional words, confirming the dissociation of the two types of the process as claimed by generative linguists. Conclusion:Based on the results patients had a better performance on derivational words as compared to inflectional ones confirming this linguistic theory that the two types of process take place in different sections, i.e. derivational process belongs to morphology whereas inflectional process is basically syntactic.