The effect of molecular weight on kinetics of thermal degradation was studied by using the thermogravimetric curves obtained by TGA instrument. ABS(acrylonitrile-butadiene-styrene-terpolymer) was used as material and its kinetic parameters of thermal degradation were calculated by Ozawa's equation. Obtained through Ozawa's method, the activation energies of thermal degradation were 70.29, 71.99 and 74.39 kJ/mol and the pre-exponential factors were 2.57×10¹¹, 4.47×10¹¹ and 7.24×10¹¹ min^-1 for molecular weights of 104200, 130900 and 141200, respectively, at a fixed degree of 0.2 conversion.