Raman spectra and infrared imaging systems are used for the study of internal temperatures of polymer light-emitting diodes. The thermal degradations of polymer light-emitting diodes with different current densities are investigated. Raman intensity is proportional to the number of molecules in the next higher vibration energy level, and accurate internal temperature of polymer light-emitting diodes at thermal equilibrium can be calculated with the ratio of anti-stokes to stokes Raman intensity by Boltzmann equation. With the current density of polymer light-emitting diodes going from 0 to 169 mA/cm2, it is found that the internal temperature of polymer light-emitting diodes increases accordingly. When the temperature comes to the glass transition temperature of the emission layer, there is a phase change in it and the layer becomes free state as liquid, which is not stable. Local disfigurement in the emission layer results in short circuit between the cathode and the anode of a polymer light-emitting diode, and the luminescence of polymer light-emitting diode fails. Therefore, Raman spectra is considered as a good method for detecting temperature of thin-film semiconductor devices.