The deformation twinning is an important deformation mechanism of magnesium and other HCP metals. This paper provides a framework of crystal plasticity model involving the effect of deformation twinning of HCP metals. The lattice rotation due to the deformation twinning is introduced into a conventional crystal plasticity framework, and the volume fraction of deformation twinning is considered. A numerical example is conducted to evaluate the evolution of volume fraction of twinned region in polycrystalline pure mangesium, and an evolution equation of volume fraction of twinning is quantitatively discussed.