Grammatical metaphor is one of the major issues in systemic functional grammar which has received much more attention recently. The main question of this research is: What are the functions of grammatical metaphors, such as nominalization and modality, in political discourse, and why do media use grammatical metaphors in texts? The main objective of this research is to study the role of grammatical metaphor in political discourse based on systemic functional grammar. In this research, the basic functions of grammatical metaphors, namely nominalization and modality, in political discourse are investigated. The needed data are extracted from Resalat and Shargh newspapers; the frequencies of nominalization in two newspapers are 408 and 304, respectively. The frequencies of modality in Resalat and Shargh newspapers are 30 and 32, respectively. The results of research indicate that main functions of nominalization metaphor are foregrounding of positive self-representation, foregrounding of negative other representation, hiding ideologies and suppression of message, creating cohesion and coherence, changing the distribution of theme and rheme, changing the distribution of old and new information. Moreover, the results indicate that the main functions of grammatical metaphor of modality in political discourse are concretizing of ideologies, expressing attitudes of writer, indicating the degree of certainty of proposition occurrence, and foregrounding ideologies.