Resumen: In this paper, we expound Francisco de Vitoria's thought about the origin, subject, nature and end of political power, how this power is natural to man, and how it acquires specific characteristics by the free decisions of men. Likewise, it is expounded Vitoria's defense of monarchy in his early Relectio de Poestate Cvili, and it is concluded that some assertions contained in that defense are incompatible with the general Aristotelian character of his political theory, and this discordance is explained by pointing out to the historical context in which this work was written.