Deformation behavior of a 5000-series aluminum alloy sheet under biaxial stress is precisely measured for linear stress paths, using servo-controlled biaxial tensile tests with cruciform specimens. Successive contours of plastic work in stress space and the directions of the plastic strain rates are precisely measured and compared with those calculated using the Yld2000-2d (Barlat et al., 2003) and Yld2004-18p (Barlat et al., 2005) yield functions. Both yield functions successfully reproduce the successive work contours and the directions of the plastic strain rates of the test material. In order to check the effect of the yield functions on the enhancement of the predictive accuracy of forming simulations a finite element analysis of hole expansion forming is performed using the yield functions. The calculated thickness strain distributions are compared with experimental ones. The prediction by the Yld2004-18pyield function using solid elements had the closest agreement with the experimental data by considering the drawbead forming process and the material deformation behavior passing over the draw-bead during the hole expansion forming process.