A rate independent crystal plasticity model is developed to analyze the plastic deformation of hexagonal close packed (HCP) materials by incorporating the crystallography of deformation twinning in plasticity model. The "successive integration method" is used to identify active slip/twinning systems and determine plastic strain. This model is implemented in the commercial finite element code ABAQUS/Explicit to simulate the deformation process of the magnesium alloy sheet. For the determination of model parameters, simulated stress-strain curves for uniaxial tension are adjusted to the measured curves. Then the stamping process of a cylindrical cup is simulated with the developed model, and the effect of initial crystal orientation on final cup earing is discussed.