Polythiophenes are of considerable interest as synthetic metals. The optical properties of polythiophenes can be easily affected by alkyl chain or other side groups to the thiophene ring. The effect of polymerization degree on the energy gap was studied in the case of polythiophene as a conjugated polymer at B3LYP/6-31+G(3d,3p) level of theory. Up to n = 16 could be confident that this degree of polymerization represents the bulk of polymer. The effect of the substitution of different groups in functionalized-bithiophene and -terthiophene on the polarizability was studied. The correlation study was performed between descriptors representing the molecular structure and refractive index for the set of 9 oligothiophenes. Three descriptor models including energy gap, polarizability and heat of formation were developed to estimate the refractive index with R2 = 0.81 and S = 0.068 by multi-linear regression analysis. The obtained model was used to predict the refractive indices of other six oligothiophenes. The model may be improved to predict the refractive index of the other oligothiophene derivatives.