Silicon-On-Insulator waveguide can be used as photoconductor, the light being coupled in the silicon film by a diffraction grating. Nonlinear effects are induced by the photogeneration of electron-hole pairs. This leads to photocurrent variations which are much faster than in linear regime and also much steeper than the incident light pulse. A model is presented which takes into account the refractive index variations arising from both the excess carrier density and the temperature rise induced by carrier recombination and Joule effect. The photocurrent is calculated for various values of the incident light power, incidence angle and pulse duration.