This paper investigates the mechanical response and the mechanisms of failure of an ultra-high molecular weight polyethylene under service at cryogenic temperature. The service temperature Ts being about 50 °C below its glass transition temperature Tg , the study focuses on the experimental techniques to determine both the glass transition temperature Tg and the ductile–brittle transition temperature (DBTT). Tg was estimated by dynamic mechanical thermal analysis (DTMA) and contrasted with the glassy–rubbery transition defined by using the Young’s modulus issued from monotonic tensile tests on smooth specimens at room and low temperature and various cross-head speeds. Concerning the DBTT, two estimators of the transition, based on the fracture surface and the up-to-failure data, were studied. An operating diagram (temperature/cross-head speed) including the probability of ductile failure and both the rubbery and glassy domains is proposed. This diagram aims at finding an optimal compromise of the material response combining stiff versus soft with brittle versus ductile behaviour.