A new quadratic planar yield function is rotated in the stress space by equivalent plastic strain has proposed. The proposed function can decrease flow stress in a uniaxial tension test for a certain direction even if the hardening monotonically increases, and is able to reproduce Lüder's band propagation and yield point elongation. The proposed function converges to Hill’s 1948 planar yield function after equivalent strain reaches a specified value. To verify the function, the DDQ steel which shows yield point elongation has been selected to compare the stress-strain curves of experimental data and finite element simulation. The proposed function reproduced Lüder's band propagation. The upper and lower yield strength of the finite element simulation fit well with the experimental data, and also the true stress-strain curve after the stress plateau followed hardening curve. It can be concluded that the proposed yield function well predicts the stress-strain curve with yield point elongation in a uniaxial tension test.