Ultra high molar mass polyethylene (UHPE) was melted at 160degreesC for various times or at various temperatures for 5 min and then the crystallization of the UHPE was carried out on cooling. It was found that the crystallization temperature decreased as the heating time or heating temperature increased. During the melting process, thermal motion of the chains leads to a change of chain conformation from parallel-extended chains to interpenetrated random coils, accompanied by the occurrence of entanglements. As a result, the crystallization temperature shifts to lower temperature. On the other hand, samples of UHPE with less entanglement were prepared from a dilute solution by a freeze-drying procedure. It was observed that the crystallization temperature of the freeze-dried samples from the melt depressed with decreasing solution concentration. UHPE would produce small crystals in the freezing process, thus leading to a reduction in melting point and a sifting of crystallization temperature to lower temperature. Based on the melting point, the average volume of small crystals was estimated; it is even smaller than that of one single chain of the UHPE.