The Rhodian Panaetius, the path-breaker exponent of the middle period of Stoic philosophy, developed his theory to justify the legitimacy of the Roman Empire before the conquered people in the second half of the second century B.C. The essence of the conception is the following: the rule of a state over another is righteous, if their relationship also serves the interest of the subject country, and this is possible if the ruling state is superior, and makes the subjugated better, governs it more adequate as if it would rule itself. The historian Polybius, who wrote about Rome’s becoming the most powerful empire of the world, knew Panaetius, they were both friends of Scipio Aemilianus, and we even know that they often discussed political questions. Even so the theory of the Rhodian philosopher does not seem to be present in the fragmentary work of Polybius — at least not coherently propounded. The thorough inquiry shows nevertheless that all substantial elements of the theory can be found in Polybius’ work. Firstly he considers justice as an objective notion (nature instructs us on it), asserts the good to coincide with the expedient, and the real expediency to differ from the seeming one: accordingly he can judge the states from an absolute point of view like the Stoics. Secondly he holds the Romans superior to the other nations with regard to their polity, military abilities and all the virtues. Thirdly he esteems the Roman governance as expedient also for the subjects and therefore worth opting for and praising, because Rome makes the citizens of the subjugated countries more temperate, religious, ingenuous, and brings peace, order and rightfulness to their public life. As the most important elements of Panaetius’ theory occur, we can assess that Polybius saw the relationship of Rome and the subject countries according to the conception of Panaetius.