With the development of ultra-fine powder technology, micron polymer materials are widely used, but the thermal decomposition hazard of micron polymers have rarely been studied. In this study, the thermal decomposition of micron Poly (methyl methacrylate) (PMMA) and Polystyrene (PS) with different sizes were studied by thermogravimetry (DTG) technique under nitrogen. Various degradation models including the Friedman method, Kissinger-Akahira-Sunose method and Coats-Redfern method were employed to determine the thermal decomposition mechanisms and kinetics of these polymers. The results showed that the thermal decompositions of micron PMMA and PS under nitrogen atmosphere follow the first-order reaction mechanism. The mean activation energy values of the micron PMMA with sizes of 5 pm, 10 Am and 15 pm were 157 kJ/mol,170 kJ/mol and 174 kJ/mol, respectively, while values of the micron PS with sizes of 5 mu m, 10 mu m and 20 mu m were 188 kJ/mol,196 kJ/mol and 206 kJ/mol, respectively. The results indicated that the thermal decomposition stability of the polymer increases with increasing particle size. This work can increase the knowledge of the thermal decomposition of polymers, in particular to understand the influence of particle size on the thermal decomposition characteristics of micron polymers. (C) 2015 Elsevier Ltd. All rights reserved.