Abstract: Keeping minor principle stress and medium principle stress fixed and loading in the direction of maxinum principle stress is a potential stress path in practical projects under which several groups of true triaxial tests about silt,fine sand,medium sand are studied.The true triaxial test results of medium sand indi cate that Poisson's ratio can be greater than 0.5 so far as to l in the direction of minor principle stress,and less than 0 in the direction of medium principle stress.These phenomena and results have not been considered in modern soil mechanics and finite element method analysis.