The dominant pathways for hydrogen diffusion in poly-Si TFT’s are identified by analyzing the hydrogenation effect on the various device geometries. It is observed that the gate poly-Si thickness and channel width didn’t affect the hydrogenation. As the channel length was decreased down to 3 µm, threshold voltage was reduced and field effect mobility was increased significantly with hydrogeantion time. In the thick gate oxide (2000 Å, 4000 Å) poly-Si TFT’s, the device characteristics have been improved rapidly with hydrogenation time. The tail state density of thin gate oxide TFT wasn’t change by hydrgenation while that of thick gate oxide TFT was significantly reduced. Our experimental results may support the model that hydrogen atoms diffuse into the bulk of the active channel layer through the gate oxide and passivate the grain boundary and intragranular defects limitedly by gate oxide area.