The legitimacy of political systems has been one of the most fundamental and disputed subjects of political philosophy among the experts. Adopting an analytic-documentary method, the paper is an illustration on legitimacy of theocracy in the “monotheistic thought” of Imam Khomeini who establishes his epistemic geometry and practical and scientific tradition by receiving inspiration form the principle of monotheism and in accordance with  the verses of the holy Quran, and for him, the rights of legislating and ruling are exclusively possessed by the almighty God so that no one can rule the people without referring to permit and command of God and all political systems lacking this support will be considered to be tyrant and illegitimate; though, from this perspective, the opportunity of exerting power for every legitimate governing system is realized only through selection and will of people.