The effect of side chains with different length on the optical properties of two PPV derivatives, poly (2,5-dioctyloxy-1,4-phenylenevinylene) and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene), was investigated by absorption spectroscopy, fluorescence spectroscopy and picosecond Z-scan technique. The optical properties of two derivatives were discussed using the theories of π-electron conjugated structure and resonant/non-resonant enhancement, respectively. The obtained results demonstrated that the absorption and fluorescence emission bands of MEH-PPV shift towards the long wavelength direction compared with the counterparts of DO-PPV. Furthermore, the side chains also affect the third-order nonlinear susceptibilities χ(3) of PPV polymers. Under the resonant excitation, the third-order nonlinear susceptibility χ(3) of MEH-PPV at 532 nm is two orders of magnitude larger than that of DO-PPV, and reached a maximum of 9.30×1010 esu.