We have analysed numerically back gate effects of a-Si TFT under illumination with a 2-dimensional device simulator. In this simulator, all localized states were taken into account as recombination centers and as charge trapping centers by Shockley-Read-Hall statistics according to the Simmons-Taylor's analysis. The simulation results explain well the enhancement of the TFT OFF current under illumination and the shoulder-like increase of the dark OFF current which were generally observed experimentally.