Hydrogenated amorphous silicon (a-Si:H) thin films, as an active layer of a hydrogenated amorphous silicon thin film transistor (a-Si:H TFT), were deposited with different amounts of hydrogen (H2) dilutions by plasma enhanced chemical vapor deposition (PECVD). The H2 atoms in a-Si:H that use an active layer are very important in improving field effect mobility (n) and in controlling the threshold voltage (Vth). The n of a-Si:H TFT is an important factor for an active matrix display device. If the n of a-Si:H TFT increases to 12cm2/Vsec, the TFT-LCD and OLED displays can have a drive IC within the panel. A high resolution display can also be made [1]. Therefore, the H2 content in a-Si:H is a key factor in the changing Vth of TFT. We carried out various studies on the properties of the materials of a-Si:H films that were deposited with a varied H2 flow rate and found that the Vth shifted from 3.6 to 1.4V with an increase in the H2 flow rate from 0 to 300sccm. The n changed also from 0.152 to 0.227cm2/Vsec.