Abstract: In this paper, Some properties of nonlinear variable structure systems （VSS） are studied, First, The synthetic method of VSS is considered, and two kinds of this controler are given. Next, in this paper more space is given to the study of properties of the sliding mode of VSS, among which the problems of the relation between vector fields of a system and its sliding mode, the properties of the VSS of mechanical systems and the pole assignment of the sliding mode of linear variable systems are concerned.