The transfer function of an EMC (Electo-Magnetic Compatibility) filter can be strongly disturbed in high frequency (HF) due to all the magnetic and electrostatic imperfections. Some of them are specific to the discrete component geometry, and the others come from stray couplings between every element of the system (the tracks as well as the components themselves). Their influences on an EMC filter response will be pointed out at first. These phenomena are commonly neglected for practical reasons. It is indeed easier to have them as low as possible by following some guidelines. The filter response is therefore the closest from the theoretical behaviour without stray couplings, but it is not the best optimisation. This paper proposes a new way to optimise an EMC filter by considering some parasitic phenomena through a new analytical modelling.