We present a new method to analyze irreversible transformation kinetics of melting in polymer crystals with temperature modulated differential scanning calorimetry (TMDSC). In the melting region of several polymers, the apparent heat capacity obtained with TMDSC can be expressed as C-s + (\F-melt\/beta)/(1 + i omega tau (beta)), with the true heat capacity, C-s, the endothermic heat flow of melting, F-melt, the angular frequency of temperature modulation, omega, and the mean time of melting of each crystallite, tau, depending on the underlying linear heating rate, beta. In the case of isotactic polypropylene, the frequency dependence cannot be approximated by this formula. The dependence suggests the possibility of the retardation in the melting kinetics to follow temperature modulation.