We report a new method for characterizing trap generation-recombination processes in Polysilicon TFTs. A small AC voltage was superimposed on a DC gate voltage and the ac current was measured at the source. A theoretical model has been developed, whereby n-channel TFTs have been analysed. A resonant peak in the imaginary part of the ac current and a corresponding step in the real part were found at the frequency of 147Hz. Our data indicate that the ac response is dominated by a single trap level. By combining the ac current measurement with low frequency capacitance measurement, we have determined the trap energy, capture cross section, trap density to be respectively 0.35eV above midgap, 3.1x10-21cm2, 5.7x1015 cm-3.