Abstract: Effect of oxygen on the thermal degradation of PMMA was studied by mass spectrometry and thermal analysis with TG and DTA. For radical polymeric PMMA, there were two main degradation models in N2: the end-chain scission model and the random chain-breaking model. The conversion rate at turning point was about 26%. Kinetic analysis results indicated the end-chain scission was controlled by diffusion process and the average activation energy E was 158.5 kJ/mol and lnA was 27.69. The random chain-breaking reaction was 1.5 order chemical reaction. The average activation energy E and lnA was 214.79 kJ/mol and 40.46, respectively. In air, there were also two stages of mass loss and the turning point conversion rate was about 70%. In the first stage, the decomposition of peroxy radicals has a notable effect on the overall degradation rate. The associated kinetic mechanism was first order chemical reaction and the average activation energy E was 130.32 kJ/mol and lnA was 24.81. In the second stage of mass loss, PMMA had already burnt and the reaction was also first order chemical reaction. The average activation energy and lnA were 78.25 kJ/mol and 13.97, respectively.