We have the Greek theory of mixed constitution in the works of Plato and Aristotle and the Roman theory in those of Polybius and Cicero. For the historian, who tries to study how the Romans, while inheriting the Greek theories, created the theories of their own, it is an important step to investigate connection between the political thought of Polybius and that of Cicero. In an attempt to trace the relationship historically, I take up the political discussions in the time of the Gracchi, which is half way between the ages of the two thinkers, considering the connection of the discussions to each of the two systems of thought. Among Polybius, Scipio and Tiberius Gracchus there is a community of thought concerning the idea of res publica and we can detect a close connection between Polybius and the Reforms of the Gracchi. However, Polybius was not fully acquainted with the originality of the Roman constitution. In the time of the Gracchan Reforms the theories of aristocracy, democracy and monarchy were discussed thoroughly and the controversy paved the way for the theory of the Roman mixed constitution. The transition from the Polybian theory of the Roman mixed constitution to the Ciceronian can be best understood as the process of the political thoughts of res publica in the Scipio-circle advancing, in accordance with the historical development of the Roman state, to the theoretical arrangement of the political thoughts in the works of Cicero, through the political discussions in the time of the Gracchi. The Polybius' theory of the mixed constitution, which remained under Greek influence and was not appropriate to the Roman reality, began to take root in the Roman world and was succeeded by that of Cicero. The fact that Cicero established his peculiarly Roman theory of state the theory of res publica-at the time of the decline of the Roman Republic forms an exact parallel of Aristotle composing the Greek counterpart the theory of πολιτεια at the fall of the Greek city-states.