The unperturbed dimensions of symmetric polymers are investigated by an improved conformational statistic method considering effect of side groups. Following the deduced formulas, the characteristic ratios of mean-square radius of gyration for poly(dimethylsilmethylene) (PDMSM), poly(vinylidene fluoride)(PVDF), polyphosphate(PP), polyoxymethylene(POM) and poyisobutylene (PIB) are calculated 0.69, 0.84, 1.23, 0.85 and 1.18, respectively. The ratios of mean-square dipole moment are 0.35, 0.48, 0.59 and 0.79 for PP, PDMSM, POM and PVDF long chains, respectively. From comparing the ratios of five polymers, it is found that the consideration of side groups for short chains is exceedingly important in calculation of characteristic quantity, especially for the inorganic polymers with heteroatom. PVDF are excellent in flexibility and electric dipole properties. The temperature characters of PIB are concerned closely with its conformational energy.