Thermal properties differences between rigid rod tetramethyl biphenyl (TMBP) and flexible diglycidyl ethers of bisphenol A (DGEBA) epoxies were studied using modified differential scanning calorimeter (MDSC), thermogravimetric analyzer (TGA) and dynamic mechanical analyzer (DMA) techniques. These epoxies were cured with phthalic anhydride (PA) and phenolic resin (PF5110), respectively. The PF5110-cured epoxy has better thermal properties than the PA-cured epoxy with a higher glass transition temperature (T-g) and a higher decomposition temperature. The good thermal properties of the PF5110-cured epoxy are due to the rigid PF5110 structure on the cured epoxy. DGEBA epoxy has a higher decomposition temperature than the rigid rod epoxy when they are cured with PF5110. But, in the PA curing system, rigid rod epoxy has a better thermal stability. These two different results are due to the different structures of curing agents.