Melting of original crystals and their recrystallization during heating were successfully separated for isothermally crystallized poly(butylene terephthalate) (PBT). The corresponding kinetics was determined and quantitatively discussed for a wide range of heating rates (0.1–100,000 K s-1) using fast-scanning chip calorimetry (FSC) and differential scanning calorimetry (DSC). The double melting peaks observed on the FSC curves are assigned to the melting of original crystals (low-temperature peak) and recrystallized and/or reorganized crystals (high-temperature peak). Heating rate dependence of the degree of recrystallization has been evaluated and the kinetics was discussed on the basis of Ozawa's method. Compared with the melt-crystallization and cold-crystallization, recrystallization kinetics is the fastest process. This is because many crystal remnants, which do not transform into the isotropic melt, act as athermal nuclei, and accelerate recrystallization.