Summary: This paper is devoted to proving some features of the non-associated flow rule such as a softening phenomenon in the stress-strain curve and the decrease of limit load. Based on the non-associated Drucker-Prager model, the analysis is performed for a soil specimen subjected to traction and compression on its edges. To obtain the stress-strain curve, we use a semi-analytical approach which provides an incremental relation between stresses and strains. The plastic limit load is calculated analytically by direct static and kinematic methods. The kinematic one is determined on the basis of bipotential concept.