Abstract: As shown in a great amount of test results and theoreticalanalyses, the direction of plastic strain increment for geomaterials isdifferent from that of stress. The direction of plastic strain increment formetals is determined by the stress on its characteristic plane--octahedralplane and it is the same as the direction of stress. Based on the analogybetween geomaterials and metals as well as the concept of the characteristicplane, the direction of plastic strain increment for geomaterials can bedetermined by the direction of the stress on their characteristic plane (SMPplane). The equations for the determination of the direction of plastic strainincrement are proposed. They can be applied to the cohesivematerials by adding a material parameter. Finally these equations arevalidated by the test data. The proposed equations are also used in ModifiedCam Clay revised by transformed stress, which can simulate the test data well.