A microthermal analysis (muTA) method with an ultrahigh heating rate was applied to a zone-drawn isotactic polypropylene fiber to evaluate the melting behaviors inherent to the microstructural changes occurring during the drawing process. The muTA melting point decreased with a reduction in the melting temperature because of the suppression of crystal reorganization and increased with an increase in the melting temperature because of the suppression of the relief of the orientation of the molecular chains, in comparison with the conventional melting point obtained by traditional differential scanning calorimetry. This study examined a convenient and powerful method for investigating the melting behavior for polymeric materials inherent to the original microstructure with less influence from thermal-measurement heating. (C) 2004 Wiley Periodicals, Inc.