Hydrogenated amorphous silicon thin film transistors (TFTs) were fabricated by plasma enhanced chemical vapor deposition on both transparent polyimide and glass substrate. The electrical characteristics and surface morphology of a-Si:H/SiNx:H films on polyimide and glass substrate were compared. TFTs fabricated on polyimide substrate show better electrical performance than on glass substrate. Moreover, the threshold voltage shift of TFTs on polyimide substrate is smaller than those TFTs on glass. The similar trend is also found either under electrical stress or 0.2% mechanical strain. The superior electrical characteristics of TFTs on polyimide substrate than that on glass substrate were attributed to the better surface morphology of a-Si:H/SiNx:H film on polyimide.