Abstract: The thermal properties, of various initial ultra-high molecular weight polyethy-lcne(UMWPE MM_w = 70-315×10⁴) powders obtained from highly active catalytic polymerization of ethylene have been studied by differential thermal analysis and differential scanning calorimetry. By plotting the ratio of △ H₁ to △H₂ against molecular weight of polymer, a linear correlation is given by Y = 0.95 + 0.28× 10^-6Mw, where Y = △H₁/△H₂ and △H₁ and △H₂ are the heat of fusion of nascent polyethylene and cooling specimen of melted polyethylene respectively.The double melting endotherms of UMWPEcan be observed when the samples crystallized from polymerization were annealed in proper condition. The melting peak of the high temperature can convert into the melting peak of the low temperature inconvertibly with different annealing time and temperature. It is reasonable to say that when the molecular weight of polyethylene formed in the polymerization system is very high (MM_w≥70 × 10⁴), the crystal chain conformation of nascent polyethylene is mainly extended and after being melted and then recrystallized on cooling, it can be converted into less extended or partially folded conformation with more crystalline deficiency.