Due to the special optical properties of cholesteric liquid crystal, it has a wide application on liquid crystal display. Experimentally, one can obtain cholesteric liquid crystal phase either by designing chiral nematic mesogen or by doping chiral molecules into non-chiral nematic phase. This thesis focuses on theoretical explanation on the helical twisting power(HTP) of chiral dopant to nematic liquid crystal. Assuming that the effect chiral dopant in nematic liquid crystal is a perturbation, we derive the chiral dopant induced liquid crystal phase deformation and its resultant helicity.