Abstract : In the objective to extend the MEMS applicative field to expensive and safety critical applications, it is necessary to predict and analyse the MEMS reliability by taking into account the conditions of use. The aim of the thesis is to propose a novel methodology for the study of the MEMS reliability through modelling. Indeed, with the realization of complete virtual prototype of the microsystem, we can not only predict its behaviour in every environmental conditions of use but also optimize its design before the fabrication. The feasibility of the methodology is demonstrated on a U-shaped electro thermal actuator as test vehicle. To demonstrate the reliability methodology (1) an analytical model has been developed for the ideal behaviour of the device using finite element method and particularly using VHD-AMS language, (2) test vehicles were fabricated and characterized to validate the analytical model, (3) ageing tests are realized to evaluate lifespan and demonstrate failure mechanisms associated with ageing, and finally (4) an empirical model has been created for ageing behaviour in order to integrate it to complete the analytical description of the device.