Exhaust systems are submitted to ever more severe service conditions, high temperature thermomechanical fatigue in particular. A numerical method is proposed for the design and the lifetime prediction of stainless steel exhaust manifolds submitted to a thermal fatigue load that is deducted from motor bench tests. The method relies on behaviour and damage models and on finite element analysis design of stainless steel exhaust manifolds. A dedicated damage criterion is proposed on the basis of the peak temperature and plastic strain amplitude reached during a thermal cycle. A more general thermomechanical fatigue damage model is also discussed.