Abstract: Dilatancy is a very important property of geotechnical materials,but now there’re different definitions about it in geotechnical engineering.In slip line theory there’re two definitions: one is the angle between the plastic strain ε and the shear strain γ,the other is the angle between the failure surface(stress slip line) and the strain slip line.In FEM analysis the dilatancy angle is defined by the plastic potential function at discretion which can only express the dilatancy property to a certain extent.The two definitions of dilatancy in slip line theory are clarified,and the generalized Mises yield criterion matching Mohr-Coulomb yield criterion in plane strain condition is put forward where the effect of dilatancy can be considered.Finally,the effect of dilatancy on the failure modes of shallow foundations is studied.