One of the most conspicuous features of the historical thought of Polybius is the fact that he places emphasis on both rationalism and the role of Tyche. In his history the word τυχη appears on no less than 137 occasions, 106 of which show his own judgment regarding Tyche. While a unanimously acceptable classification is impossible, it would appear that the word is used 13 times to mean Providence or the purposive power working toward a definite goal, and 31 times to indicate the capricious power working for instability in human affairs. He admits in several passages that chance(Tyche) could contribute to history, but in other passages insists that to talk of chance is a poor explanation. There has been much discussion as to why he was inconsistent regarding this matter. We consider that it might be understood if we look at his political career. He saw too many cases where fortunes were suddenly and sensationally reversed, the cause of which it was impossible to explain. However, to acknowledge the power of Tyche is not to give up every effort. He insists that those who engage in politics should do their best with sound judgment and rational calculation. Here he stands within the Greek tradition of rationalism. To make this point clear, we tried to compare Polybius with Ssu-ma Ch'ien, a Chinese historian who was born half a century later. We can find many parallels between them. Ssu-ma Ch'ien regarded the study of the past as essentially a means of attaining practical ends by learning lessons. In a short essay at the end of the Basic Annals of Emperor Kao-tsu, he characterizes the governmental principle of the first three dynasties, the Hsia, Shang and Chou, and demonstrates the cyclical theory of history. He makes frequent references to the so-called "Mandate of Heaven", but in some cases expounds the opposite view that men's failures are due primarily to their own faults and may not be attributed to Heaven. It has of ten been noticed that he was afflicted by the problem of the caprice or injustice of Heaven, and he wrote the famous Memoir on Po I and Shu Ch'i, which reminds us of the episode in Polybius(XXXII, 4,3). However, we cannot find such rationalism as Polybius' in Ssu-ma Ch'ien. His Shih-chi or Records of the Historian was intended primarily as a guide to moral conduct. He emphasizes the importance of virtue or high-mindedness, and writes as if most events were caused by psychological motives. We also noticed many accounts about the movements of stars, and pointed out that those of Polybius are astronomical while those of Ssu-ma Ch'ien are astrological. The contrast between them shows that they stand within different traditions.