Ki is the important conception of Chinese philosophy, many of philosophers have discussed it. So the meaning of it is various in accordance with the times and the scholars. And it has been discussed not only in the sphere of philosophy but medicine, astronomy, g eography, society and culture ; therefore we can't understand the Oriental studies without the comprehension of it. Above all, the Oriental-medicine is based on the principles of it, that is to say 'Ki-medicine'. There are Ki of a letter, Ki of a philosophy, Ki of a nature and Ki of the human body according to the recognition process from that time on the ancient. This paper is written with the view of the right understanding about it by the comparison of various theories dealed with it.