We measured the relaxation time of bend distortion in a nematic phase by using light scatterring techniques for various liquid crystal mixtures having phenylene or cyclohexylene group in the core. A cyclohexylene group gives rise to faster relaxation of bend, whereas the relaxation times of twist and splay depend little on the core structure. In addition, the relaxation time of bend mode was twice or three times shorter than that of twist distortion for the cyclohexylene compounds. These results suggest that the responce time of an LCD can be improved by using bend distortion of liquid crystals having a cyclohexan in the core.