Photoluminescence properties are studied in Poly[(2-methoxy,5-octoxy) 1,4-phenylen evinylene] (MO-PPV) thin film,which are prepared from MO-PPV chloroform solutions of different concentrations. Photoluminescence,excitation photoluminescence and selective excitaton photoluminescence measurements at room temperature have been perfomed. A long wavelength emission component near 630nm has been observed and identified through the results of experiment associated with the curve fitting method. The analyses results show this long wavelength emission peak should be attribued not to the emission from aggregators or excimers in films but to the vibronic transition from S₁ excited state to S₀ ground state. The variation of the emission peaks in thin film samples of different concentrations would be another proof to this conclusion and be explained by means of the integral strength of the Frank-Condon factor.