The glass transition temperatures and thermal stability of ternary systems containing epoxy, PMMA grafted natural rubber and nanoclay were studied. Natural montmorillonite (Cloisite Na) and chemically modified montmorillonite (Cloisite 30B) were used at 5 phr, while the amount of rubber was fixed at 5 phr. Poly(etheramine) was used as the curing agent at the equivalent and 1.05 amine-epoxy molar ratios. Mechanical stirring and ultrasonic mixing were used to achieve dispersion. Both unmodified epoxy/clay nanocomposites and cured toughened epoxy/clay nanocomposites were evaluated using differential scanning calorimetry (DSC) and thermal gravimetric analyser (TGA). The use of 2 to 7 phr nanoclay in epoxy was found to decrease the glass transition temperatures. The significant reduction could be observed for nanocomposite system cured by above-stoichiometric concentration of curing agent, followed by organoclay system. The use of unmodified clay seems to be capable of improving the thermal stability of epoxy, both in binary and ternary systems.