The mechanical 7 relaxation of a linear polyethylene has been extensively analysed by means of high resolution mechanical spectrometry. This analysis was completed by quantitative measurements of the morphological parameters governing the characteristics of the 5, relaxation, such as the crystallinity index and the crystalline lamellar thickness distribution determined from differential scanning calorimetry. The 7 relaxation was found to have the characteristics of mechanical relaxation due to glass transition: there is a high apparent activation energy; linear polyethylene exhibits structural recovery induced by annealing at a temperature below the )' peak, whereas the 7 relaxation has a WLF temperature dependence. As for the i' relaxation, it was found that the/3 relaxation displayed by a branched polyethylene has the characteristics of a glass transition (high value of apparent activation energy; structural recovery during ageing at T < Tt~). Recalling definitions of the glass temperature, it was proposed that the lower glass transition (7 region) is characteristic of Van der Waals interactions between CH 2 groups while the upper glass transition ({/ region) is specific of Van der Waals interactions between CH 2 and CH 3 groups.