In view of emerging applications in aeronautics, low temperature and nuclear fusion technology, the mechanical properties of plastics have to be tested both at cryogenic temperatures and under certain radiation environments. In the present paper measurements are reported of the elastic modulus, the ultimate tensile strength (UTS) and the failure strain of a polyimide (SlNTIMID), which were carried out at room temperature, 77 and 4.2 K. In addition to this temperature dependence, the influence of sample size on the mechanical properties, as well as the fracture behaviour in mode I on cylindrical precracked samples with a circumferential notch, have been investigated. The results show an increase in both the elastic modulus and the UTS by 40 and 60%, respectively, on decreasing the temperature to 77 K, but no further change at lower temperatures. On the other hand, the failure strain decreases continuously (by = 25%) down to 4.2 K, while the fracture toughness increases in a similar way (by = 10%). No sample size dependence could be detected. These results will be discussed in conjunction with additional observations of the fracture behaviour made using optical and scanning electron microscopes.