Although it is expected that, the C=C bond strength will increase after introducing the cyano side-group, thus its photo-stability should be improved. But after careful study, we found that when exposed to light, the introduction of cyano group makes the new molecule alpha'-cyano-4'-nitro-4-N, N-dimethylaminostilbene (CNDS) to isomerize much easier than 4'-N,N-dimethylamino-nitrostilbene DANS. SHG, FTIR, UV-visible absorption and optical pump induced anisotropic absorption measurements were done to support our conclusions. Careful study of the photoinduced optical anisotropy(POA) shows that the isomerization of CNDS is a little smaller than the azo-dye molecule dimethylamino-nitroazobenzene (DMANAB). Explanation was given based on trans-cis quantum yields, de-planar molecular structure which induces small order of orientation and slower thermal relaxation.