Isothermal uniaxial cyclic loading experiments have been conducted on specimens of the nickel-based superalloy IN100 at selected temperatures in the range 600–1000°C. Power-law stress—strain relations based on plastic strain (Ramberg-Osgood), total strain and their bilinear equivalents have been used to describe the behaviour. In addition, uniaxial thermomechanical fatigue (TMF) tests have been performed on tubular specimens of the material over the temperature range 650–1050°C. The aim of these tests was to model the response during thermal shock experiments involving rapid heating and cooling cycles between 200°C and 1100°C. Some transient tests have also been carried out on blade-shaped single-wedge specimens of IN 100 to simulate the type of thermal fatigue (TF) cycle experienced by turbine blades during service. A finite element sensitivity analysis has been undertaken to predict the results of both the TMF and TF tests using the two different bilinear descriptions of the isothermal cyclic stress-strain response of IN100. Good agreement between the calculations and the TMF experimental results is obtained whichever bilinear relation is chosen.