Melt, cold isothermal crystallization kinetics, and multiple melting phenomena are investigated by differential scanning calorimetry (DSC) for a flame-retardant phosphorus containing copolyester. The crystallization kinetics was investigated by the Avrami equation. The Avrami exponent is about 2.6 for melt crystallization and about 2 for cold crystallization. The crystallization activation energy for melt crystallization and for cold crystallization is -64.7 and 145.5, respectively. Three melting endotherms are found in the DSC scan, and they are explained in terms of secondary crystallization, primary crystallization, and recrystallization during the scan. A strong evidence of a two-stage crystallization mechanism was also observed in the DSC isothermal experiment and X-ray diffraction.