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Anisotropic yielding and hardening behaviors are important parts of a constitutive model for material formability and strength assessment. In this work, the work hardening characteristics of a high strength steel sheet after it was subjected to 2% and 5% plastic strain in rolling and transverse directions were investigated by both the experimental measurement and mathematical modelling. It was observed that the material with the mild initial anisotropy showed a strong anisotropic work hardening effect after strain path was changed. A material model was proposed by Ma and Aung in order to reproduce the observed hardening phenomena under various conditions of strain path change. The proposed formulation has a simple form mathematically derived from a modified Swift law which can be easily implemented into existing FEM software as a user material model. This anisotropic hardening model for plane-stress condition was combined with Hill48 yield function and associative flow rule for numerical simulations. The model was validated through comparing the simulation results with experimentally measured stress-strain curves.