Making a comparative study between MOSFET and polycrystalline silicon thin film transistor(poly-Si TFT) in applying the charge pumping method(CPM), the most distinctive difference was found in the relation of the charge pumping current(Icp) versus the gate pulse frequency. According to the conventional theory, Icp increases linearly with the gate pulse frequency. In poly-Si TFT, however, Icp shows complicated dependence on frequency. We modeled the frequency dependence of Icp in poly-Si TFT by considering the resistance of active poly-Si film. According to this model we can extract the parameters such as grain boundary trap density, substrate resistance, and capture cross section.