Abstract: On the basis of biaxial tensile experimental system, twotypes of steel sheet were studied under different loading paths usingoptimized cruciform specimens with slots in the arms. According to theexperimental true stress-logarithmic strain curves, the stress-strain curvesof steel sheet vary with the stress state, and the strain-hardening exponentbecome larger when the load ratio is near the ratio 1:1.The contours of experimental yield loci for two steel sheets from 0.2\% to2\% plastic strain were determined based on the equivalent plastic workprinciple. It is found that the shape of yield loci does not show symmetryalong equibiaxial stretching path because of anisotropic mechanicalproperties.The geometry of the experimental yield loci were compared with the yieldloci calculated from several existing yield criteria and the analyticalresult shows that the Hosford yield criterion describes the general trendsof experimental yield loci of steel sheets very well, whereas the Hill48yield criterion overestimated the yield stress in all the contours. Inaddition, the Mises yield criterion that is generally used in isotropicmaterials also gives good fitness to the experimental yield loci.