The commonly accepted method ASTM E 698 for determination of the kinetic parameters of a thermal decomposition process was critically evaluated. In order to show possible errors in application of the method, which the weakest points are problems in deconvolution of TG and DTG curves and determination of T-max, several numerical simulation of TG curves have been performed. Various deconvolution techniques such as Gauss, Cauchy, pseudo-Voigt, Fraser-Suzuki, and Pearson techniques were employed. The results of the simulations revealed that the best fitting to the DTG curve for the first order reaction gives the profile of Frasera-Suzuki, but the important sources of errors in estimation of the activation energy E and pre-exponential factor is not enough precise determination of T-max.