A series of alpha,alpha’-thiophene oligomers and their alpha,alpha’-substituted derivatives such as formyl, bromine, and cyano have been prepared in order to discuss the effects of number of thiophene rings and introduction of functional groups on third-order nonlinear optical properties. Optical absorption spectra and optical third harmonic generation (THG) have been studied in vacuum-evaporated thin films or solid solution. The values of lambda max, linear polarizabilities and chi((3))(-3 omega;omega,omega,omega) obtained from THG measurement for thiophene oligomers increase with increasing number of thiophene rings. The introduction of electroactive groups and increasing the molecular planarity enhanced the linear polarizabilities and third-order nonlinear optical susceptibilties.