Thermal stability of specimen cured at various mixing ratios of epoxy resin to acid anhydride in the presence of an accelerator was investigated by means of differential scanning calorimetry (DSC), thermorgravimetric analysis (TGA), and Fourier transform infra-red spectroscopy(FTIR). The mixing ratios, represented by weight ratio (A/E=anhydride/epoxy), were epoxy-excess (A/E=0.6, 0.8), stoixhiometric (A/E=1.0), and anhydride-excess (A/E=1.2). In this study, the mixing ratio greatly affects the maximum glass transition temperature ($T_{g}{infty}$) and weight loss initiation temperature ($T_{i}$). Among the mixtures, an epoxy-excess ratio, A/E=0.8, showed the highest $T_{g}{infty}$ and $T_{i}$ which suggests it is the most thermally stable. The thermal stability of the specimen may be attributed to i ) the formation of additional ether linkages which are known to have better thermal resistance than esters, ii ) the increase in the crosslinking density, and iii ) less unreacted epoxy and anhydride groups compared to other ratios. Molecular structure of cured specimen for various mixing ratios is discussed.