Abstract: in this paper, first of all, the concept of suction is generalized to include all factors which can increase the resistance against sliding between soil particles, and a principle of generalized effective stress is proposed for unsaturated soils based on this generalized suction concept. Next,assuming that the void ratio Of saturated disturbed soil sample is uniquely determined by the stress state and defined as stable void ratio, a stable state theory is formulated which POstulates that during the loss of generalized suction the void ratio tends to stable void ratio, i. e.the under-compressed soils contract but the over-compressed soils expanse. A constitutive model enable to simulatethe process of loss of generalized suction is suggested, and the behaviours of a collapsible soil and an expansive soilunder loading and/ or wetting are analyzed with this model.