A method for fabricating the polycrystal silicon TFT under a low temperature which has an improved electron mobility, comprises the steps of forming an oxide film on a substrate, depositing a polycrystal silicon on the oxide film and patterning the polycrystal silicon so that source and drain regions and a channel region remain, growing a gate insulating layer on the patterned polycrystal silicon by ECR plasma thermal oxidation, depositing a material for a gate on the whole surface and removing the material and the gate insulating layer in portions except for a gate region to form the gate, and performing ion implantation on the exposed areas of the polycrystal silicon to form the source and drain regions.