Thermal decomposition behaviors of a fluororubber are observed with linear heating mode at constant rate, controlled rate mode and isothermal mode of TG/DTA, and, with isothermal mode of TG/mass-spectrometer. The results suggest that this material decomposes with, at least, three consecutive reactions. From the results of TG/DTA and TGMS in isothermal mode, it is considered that the first reaction is the first-order reaction, the second reaction is chain reaction and the last reaction is vaporization of residue. Supposing these three consecutive reactions, the decomposition behaviors of the rubber is simulated. The observed curves of fraction of mass loss, alpha, against time, t, are well reproduced by the simulation. The relationship between d alpha/dt and alpha is reproduced also, though the differences between observed and the calculated values of d alpha/dt are slightly larger than noise level of d alpha/dt.