The accurate knowledge of the effective channel length in a polysilicon thin film transistor (TFT) is of great importance. The reduction Delta L of the channel length is due to lateral diffusion from the source and drain contacts into the gate region. The usual method for determining Delta L is based on plotting the straight line 1/1(D) versus length mask L-m in the linear region and extrapolating the straight line to intersect the x-axis. We show that this method is approximate and leads to an underestimation of the reduction channel length due to the existence of the access resistance. We propose an alternative method based on the saturation current I-Dsat using the 1/I-Dsat versus L-m plots. We show that this method is more accurate especially when the access resistance of the device is important. The extracted parameters Delta L for n- and p-channel polysilicon TFTs are more compatible with the device performance.