This paper presents an innovative bench specifically dedicated to high RF power device lifetime tests under pulse conditions for radar application. This bench is able to stress eight devices in order to have a statistic sample for MTTF estimation. The bench allows to apply and keep track of all RF powers, voltages and temperatures. Both electrical and thermal stresses applied should lead to modifications of the device performances. A commercial LDMOS transistor has been chosen for the first tests. This paper presents the electrothermal modelling performed to understand the degradation phenomenons that appear during device ageing. The model will be used as a reliability tool in order to correlate RF LDMOS electrical parameters with any kind of degradation phenomenon.