Dynamic mechanical thermal analysis (DMTA) has been applied oil medical grade ultra high molecular weight polyethylene of different crosslink density as prepared by electron beam irradiation to probe for contributions from crosslinking as well as crystallization. The crosslinking proceeds at a crystalline structure with a crystallinity about 50%. With increasing, irradiation dose from 0 to 110 kGy, the molar mass between adjacent crosslinks decreases significantly to reach 3170 g/mol at lowest, whereas the crystallite thickness changes and new thin lamellae grow at almost constant degree of crystallinity. From DMTA in the entire temperature range from -150 to +140 degrees C and the angular frequency range from 0.6 to 135.4 Hz, three relaxation processes gamma, beta, and alpha of different temperature position and activation energy are distinguished. The corresponding chain mobility has been discussed as sensitive discriminator for the coexisitng crosslinked and lamellar phases showing the same dimension of a couple of 10 of nanometres. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 49-59, 2009