Molecular-length dependence of the third-order nonlinear optical properties in conjugated organic materials is systematically investigated by third-harmonic generation measurement and optical Kerr shutter measurement. The molecular length represents a very important factor in the third-order nonlinear optical properties. The third-harmonic generation measurement shows that the nonlinear refractive index related to the electronic polarization effect increases in proportion to the powers of a conjugated length or an absorption edge. The optical Kerr shutter measurement indicates that the application of longer molecules causes the nonlinearity of the solution medium to be dominated by the electronic polarization effect, reducing the contribution of the molecular orientation effect.