Although no scholar in classics today dare to claim that ‘rationality’ is the only characteristic of ancient Greek thinking, it is undeniable that ‘rationality’ is still one of the main key words that characterize the ancient Greek thinking. The ancient Greek medicine was also regarded as the field where rationality was expressed in an exemplary way. This paper aims at presenting how different stages of rationality appeared in history from the classical antiquity to late antiquity. The Hippocratic medicine stands out as the model of rational medicine in Western medicine. But it is not easy to single out the type of rationality that the Hippocratic medicine represents since the Hippocratic corpus has been formed over hundreds of years by many different authors with different views on medicine. Speculative as well as empirical characteristics coexist in Hippocratic medicine. However, speculative rationality characterize the medicine after Hippocrates. A lot of medical schools, which was called dogmatists, asserted their own opinions on human physiology and pathology. The famous Alexandrian medical scientists Herophilus and Erasistratus were regarded as the model of speculative rationality in medicine. In reaction to the speculative rationality appeared empirical rationality. The Empirical school denied the validity of speculative knowledge and acknowledged only the knowledge confirmed by experience, which lead the emerging of Scepticism in medicine and philosophy. Finally, Galen attempted to conciliate these two opposing opinions on medicine. Galen adopted a new strategy of stressing rationality in methodology, which prevented him from inclining either speculation or experience alone. His balanced attitude towards medicine had probably made his medicine to last a thousand years to come.