A technique is presented to extract the threshold voltage of amorphous thin film MOSFETs in the saturation region. The technique is proposed because threshold voltage extraction in amorphous TFTs is different, and in general more complex, than in conventional crystalline bulk devices, since these TFTs exhibit several notable dissimilarities inherent to their characteristics. The saturation drain current follows an m power-law type dependence on gate bias, with an m different from the conventional value of 2. Additionally, a plot of the saturation current as a function of gate bias does not reveal the existence of an inflexion point. The method presented, which extracts the value of the power-law parameter rn as well, is based on the use of an auxiliary operator that involves the integration of the drain current as a Function of gate voltage. The technique was tested and its accuracy verified using the measured characteristics of an experimental n-channel a-Si:H thin him MOSFET.