A simplified and novel kinetic model was first developed by way of characterization of Fourier transform infrared spectroscopy, scanning electron microscopy, and the weight-loss method for PMMA decomposition under a microwave assisted digestion system. By Newton's method, the experimental results of the remaining weight of PMMA are closely fitted by the model combined with zero-order and first-order kinetics, in which the former predominates the reaction at lower temperatures (423-443 K) and the latter at higher temperatures ( 453 K). Kinetic parameters of PMMA decomposition under 423-453 K including rate constants and the, mass fractions (phi) via main-chain scission were determined by this empirical model. Activation energies of PMMA decomposition estimated by the Arrhenius equation are 0.74 and 36.07 kcal/mol respectively for the zero- and first-order reaction. The pre-exponential factors of the zero- and first-order reactions are 2.28 x 10(-3) and 2.57 x 10(17) g min(-1) respectively. The effect of HNO3 volume on PMMA decomposition was further investigated at 423-473 K. At 473 K, the digestion efficiency has increased to 100% as the HNO3 volume is >= 3 mL. The estimated phi values of the decomposition are increasing with the HNO3 volume at 423, 443, and 453 K, yet varying insignificantly at 473 K.