The effects of sample size and heating and cooling rates on thermal transitions of ultrahigh-molecular-weight polyethylene (UHMWPE) were investigated. The thermal parameters were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). While heating rates and sample size had strong effects on thermal properties, the influences of cooling rates were minor. For DSC, broad melting transitions were obtained at faster heating and/or slower cooling rates and larger sample sizes. Higher melting temperatures were obtained when heating rates and sample size were increased. Slower cooling rates also produced higher melting and crystallization temperatures. Faster cooling rates yielded lower heats of fusion during melting and also lower heats of crystallization. The dependence of peak melting and crystallization temperatures on the heating and cooling rates are illustrated by two empirical formulas. For TGA, it is found that faster heating rates and larger sample sizes produced higher decomposition temperatures. This detailed analysis may explain the large variations in the reported data on thermal properties and crystallinity of UHMWPE and provide solutions to the current clinical problems associated with polymeric biomaterials.