The terpolymer resins have been synthesized by the condensation of 2,4-dihydroxypropiophenone with biuret and formaldehyde in the presence of 2M HCl as catalyst with varying proportion of reactants. Thermal decomposition curve are discussed with careful attention of minute details. The Freeman-Caroll and Sharp-Wentworth method have been used to calculate thermal activation energy and thermal stability. Thermal activation energy (Ea) calculated with these methods are in agreement with each other. Thermodynamic parameter such as free energy change (DF), entropy change (DS), apperant entropy change (S*) and frequency factor, (Z) are also determined on the basis of TG-curve and using data of Freeman-Caroll method. The order of thermal stability of terpolymer has been determined by using TGA.