Abstract: Taking Systemic Functional Linguistics and Appraisal Systems as its theoretical framework, this paper explores deontic modality and its interpersonal meanings in political speech from the perspective of van Dijk’s model of ideological analysis. Based on statistics and illustration of 30 Obama’s Weekly Addresses, this study finds that the speaker prefers obligation, high level and subjective deontic modality devices, which have multiple interpersonal meanings, namely, expressing attitudes, satisfying social psychological strategy. And above all, deontic modality in political speech is the token of engagement of interacting stances and voices, uses power, manipulates the public, and spreads the ideology of the speaker and the political party the speaker represents.