Abstarct: In this thesis, we will be study controllability, observability and stability of descxriptor systems. By using orthogonal equivalence transformations, we will introduce condensed forms, that will show the regularity of the system, and the from this regularity, we will study the controllability and observability of it. Because working with a standard system is much easier than the descxriptor system, so by using the derivation state feedback and proposition the descxriptor system is changed into a standard system. In this thesis a new method for calculation the linear parameter state feedback matrix is introduction by using the similarity transformations. Then state feedback matrix with minimum norm will be determined in such a way that the system remain stable.