The purpose of this paper is to assess the fatigue strength of complex structures submitted to low cycle thermomechanical fatigue. In a first step, the behaviour of the material in a high range of temperatures has been modelled using a simple elastoviscoplastic constitutive law. In a second step, a criterion based on the dissipated energy per cycle derived from the results of the numerical computations, allows the prediction of the fatigue strength of the structure. Strain controlled isothermal low cycle fatigue tests were carried out on cast iron specimens for the identification of the constitutive and the fatigue laws. Temperature controlled tests on specimens and structures permitted the evaluation of the precision of the numerical computation under anisothermal loading. The lifetime prediction is showed to agree with the experiments both on specimens and structures.