Abstarct: Toadays, polymers with a lot of qualifies and extended applications impress a lot of aspects of our life. Conducting polymers have attached much attention as advanced materials. In these decades, the widely studies have been done in exploring new capabilities of these materials with their optimizing and new uses. In this thesis, we attended to polythiophene as a conducting polymer and investigated the influence of the nature of the alkyl substituents on the structural and electronic properties of oligothiophenes. We carried out calculations with the ab initio and the density functional theory methods. The geometries of all of the monomers and oligomers studied were optimized by 6-31G** basis set. Our calculations on the monomers have shown that structure parameters such as the bond length of monomers vary in the present of the alkyl group but it is not affected with increasing length of the substituent. Also, the substituent effect caused charge equality alert in α, α´ and β, β´ ring positions. Charge density increases in positions near to the substituent and decreases in positions far from the substituent. The study of monomers indicates that introduction of substituting alkyl to thiophene reduce their energy gaps and these will be smaller with increasing the substituent length. But it is not different with large length from propyle group. So we selected methyl, ethyl and propyle substituents to continue our study on the oligothiophenes under optimum calculations, the HT-HT regioregular chain used for study of of oligomers. It is found that the radical cation oligomers have the better conditions for conducting than corresponding neutral form because their conjugated length approach to same value and have not observed any torsion angle in chain polymer which play important role for conducting in conjugated polymers. Our studies reveal increasing chain length of oligomers leads to smaller gap. However, some of differences observed in the energy gap of short chain oligomers, but these reduce when the chain length of oligomer increasing. It is found that type of alkyl substituent does not considerable role on the energy gap of large length oligomers. Also, our studies on ionization potential indicated that ionization potential decrease when the length of oligomer chains increases.