We report the temperature-dependent photoluminescence (PL) behavior of two poly (p-phenylene-vinylene) derivatives with different backbones, a homopolymer MEH-PPV and a copolymer MEH-OPPV. The PL peak positions of the MEH-PPV in both solid solution and film are blue shifted with increasing measurement temperature because the thermally induced torsion and libration modes reduce the conjugation lengths of the polymer, but the shift rate is smaller in the MEH-PPV film than that in the solid solution. The PL peak position of the MEH-OPPV film is independent of measurement temperature because there is a large dihedral angle between the adjacent monomer units. The large dihedral angle increases the conformational disorder to eliminate the effect of the disorder induced by the thermally induced torsion and libration modes on the conjugation lengths of the MEH-OPPV.