The conventional isoconversional plots, In beta versus 1/T and In(beta /T-2) versus 1/T. have some errors in calculating the activation energy due to the use of an approximate value of p(x). In order to obtain the 'exact' value of activation energy of a reaction, an iterative procedure has been established after the ratios, H(x) and h(x), were introduced to determine 'exact' p(x) instead of the approximate values of p(x) used in the conventional isoconversional plots. The iterative procedure is wide applicable, no matter how little or how great the E/RT value of the reaction is.