Abstarct: In this thesis, we attended to polythiophene as a conducting polymer and investigated the influence of closing the end-side of oligothiophenes chain by phenyl group on the structural and electronic properties of oligothiophenes. The geometries of all of the studied oligomers were optimized at the DFT-B3LYP/6-31G** level of theory. Results of calculations on the structure of oligothiophenes show that, in phenyl-capped oligothiophenes, the torsional angles are smaller and their structures are close to planar. Length of single bond and double bond of conjugated system of phenyl-capped oligothiophenes are closer to each other and have a better condition for charge delocalization. Energy gap in phenyl-capped oligothiophenes is reduced in compare with corresponding oligothiophenes and cause to easy charge transfer along the oligomeric chain. Results of charge carrier injection rate show that, electron and hole injection rates of phenyl-capped oligothiophenes are more than corrosponding oligothiophenes. Spin density distribution along the conjugated chain of phenyl-capped oligothiophenes have a fewer changes, which cause to better delocalization along the conjugated chain. Studies on electronic properties of oligomers show that, with increasing the length of oligomers chain, conducting properties of these materials is improved. Also the results show that, with increasing the length of the oligomers chain, the effect of phenyl-capped group is decreased.