We present a non-linear model of a Fabry-Perot resonator where the stored power can heat the mirrors and induce thermal aberrations, which, in turn, change the resonant wave. We first describe a sequential modelling of the temperature evolution, thermal aberrations and stored power; this model is then implemented with the help of a numerical propagation code. We give numerical examples in the field of cavities involved in gravitational wave interferometric antennas, for which thermal problems may arise and restrict their sensitivity.