The conventional isoconversional plot of ln(beta/T-2) versus 1/T usually has a large error of about - 10% in calculation of the pre-exponential factor as a consequence of assuming h(x) = 1. Iterative calculation using the plot of ln(beta/hT(2)) versus 1/T is thus suggested. It is illustrated that the iterative method can greatly improve calculation for the pre-exponential factor. Success in calculation for the pre-exponential factor is accompanied by an enhanced accuracy in value of activation energy given by the iterative method in comparison with the conventional isoconversional plot of ln(beta/T-2) versus 1/T. One explanation is due to the compensation effect of the Arrhenius parameters. The iterative method has been applied to thermal degradation reactions of poly(methyl methacrylate) (PMMA) and polyethylene. (c) 2006 Elsevier Ltd. All rights reserved.