ουσια is the most important concept in Aristotle's philosophy. However it is not so clear that it obviates all other difficulties. For example, the first problem is the argument about the subjective character and the verbal character of ουσια. The second is the argument concerning the priority between η πρωτη ουσια and η δευτηρα ουσια. And then, the word η πρωτη ουσια expresses a different meaning in Categoriai and in Metaphysics. In Categoriai it means το τοδε τι, but in Metaphysics it means το ειδοζ. Therefore the interpretation becomes difficult. The third is the argument regarding the inner structure of ουσια and the conditions of the real ουσια which Aristotle says. It is sertain that Aristotle does estimate positively the role of η υλη which is different from other, greek philosophers. In this paper I will make concrete some points according to the analysis of ουσια in Aristotle.