The Copolymer (SATF) was synthesized by condensation of salicylic acid and thiosemicarbazide with formaldehyde in the presence of 2M HCL as a catalyst at 126 ± 2 0C for 5 hrs. with molar proportion of reactants. The copolymer (SATF) was characterized by elemental analysis, FT-IR, UV-Visible 1H-NMR Spectroscopy. The thermal decomposition behavior of SATF copolymer was studied by using TGA in static nitrogen atmosphere at a heating rate of 100C/min. Freeman Carroll and Sharp-Wentworth methods were used to calculate the thermal activation energy(Ea), the order of reaction(n), entropy Change (ΔS), free energy change (ΔF), apperent entropy change (ΔS), and frequency factor (Z). The order of decomposition reaction was found to be 0.9. The thermal activation energy determined with the help of these method was in good agreement with each other.