We have examined the applicability of a new analysing method of temperature modulated differential scanning calorimetry to the exothermic process of poly(ethylene terephthalate) crystallization. The method utilizes the change in the phase lag between modulation components of sample temperature and of heat flow, to introduce an apparent heat capacity of complex quantity. The phase lag showed a peak and a dip during the isothermal crystallization, above and below the temperature at which the growth rate of crystals becomes a maximum, respectively. The present method incorporates the change, and predicts negative and positive temperature dependence of crystal growth rate, for the peak and dip in the phase lag, respectively. The temperature dependence of crystal growth rate agreed well with the literature values obtained from the direct measurements of growth rate of spherulites by optical microscopy.