The strong correlation of melting behavior with the microstructure of original and zone-drawn isotactic polypropylene fibers was evaluated by microthermal analysis (micro-TA) combined with wide-angle X-ray diffraction analysis. The crystal structure of both the original and zone-drawn fibers was a monoclinic a-form with a similar to 0.64 nm, b similar to 2.03 nm, and c similar to 0.65 nm. In contrast to the absence of any oriented polymer molecules in the original fiber, the polymer molecules in the zone-drawn fiber were extended and highly oriented in at least two different states; one arranged along the drawn axis and the other at 40 degrees to the drawn axis. The micro-TA-derived melting points corresponded to these microstructural changes caused by the zone-drawing process and differed from the melting points obtained by differential scanning calorimetery. The micro-TA melting point of 140 degrees C corresponded to the z-e-p melting point of the original fiber, being much lower than the conventional differential scanning calorimetery melting point of 163 degrees C. As for the zone-drawn fiber, an increase of 19 degrees C in the melting temperature revealed its high orientation level, and the appearance of three elevated melting peaks (161, 167, and 178 degrees C) at the highest heating rate (1500 degrees C/min) coincided with its several oriented states.