The mechanical & numerical model for deformation behavior of unsaturated soil is proposed, where the probability theory and statistics are applied to estimate the mechanical behavior at the contact points in unsaturated soil. Adding the consideration on the change in contact angle, i.e., probabilistic motion of soil particles, the energy surface which has been a key physical factor is found not to be needed in the revised model. The numerical experiments are carried out to compare the stress∼strain relations with those obtained from the triaxial compression tests on unsaturated sandy soil under drained condition. It is found that the revised new model can express the deformation behavior of unsaturated soil and is promising.