Three‐dimensional computer calculations which analyze time‐transient behavior of heat conduction and phase‐change kinetics in an InSbTe overwritable disk were made. To improve the calculation accuracy, thermal conductivities of thin‐film materials and critical cooling rate for crystallizing the recording film were estimated. For realization of high‐performance overwrite, it was found that the cooling conditions of writing and erasing processes should be controlled to match the critical cooling rate in a given linear velocity range. This could be achieved by optimal design of the disk structure.