Thermal decomposition of Nylons in helium gas atmosphere was studied by EGA-EI (electron ionization) MS and EGA-PI (photoionization) MS. The EI mass spectra were difficult to characterize the decomposition products due to the overlapping of fragmentation ions. On the other hand the PI mass spectra characterized clearly the decomposition products by the parent ions only. It shows that EGA-PIMS is a useful method in the measurement of thermal decomposition of polymers.