The crystallization behaviors of ultra high-molecular weight polyethylene (UHMWPE) in liquid paraffin under isothermal and nonisothermal conditions were studied by differential scanning calorimetry during the solid–liquid thermally induced phase separation (TIPS) process. For isothermal crystallization, the development of relative crystallinity with the crystallization time is analyzed by the Avrami equation with the exponent n = 2.7. The relatively high content of secondary crystallization at higher crystallization temperature can be obtained due to the high mobility of UHMWPE chains. For nonisothermal crystallization studies, the Avrami theory modified by Jeziorny is used, and the result is found that the Avrami exponent n is variable around 5 and decreases slightly as the cooling rate decreases. In addition, the extent of secondary crystallization increases with increasing cooling rate. The calculated activation energies are 881 kJ/mol for isothermal crystallization obtained from the Arrhenius equation and 462 kJ/mol for nonisothermal crystallization from the Kissinger equation, respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.