Thermal effects on optical properties and non-linear optical absorption have been studied in hydrogenated amorphous silicon films using dual beam thermal lens technique. A change of the thermal lens signal, from positive to negative, with increase of the excitation power at some wavelengths, has been observed. The thermal conductivity and thermal diffusivity were K=(2.6±0.1)×10 −3 W/cmK and D=(1.9±0.1)×10 −3 cm 2 /s, respectively. Moreover, we observed a decay in the transmission signal, which was fit with two exponential functions, related to thermal effects, and competition between the creation of photo induced defects and their thermal annealing.