Thin film transistors (TFTs) using In-Ga-Zn Oxide (IGZO) as active layer and the gate insulator was treated with NH3 plasma and N2O plasma, respectively, which is fabricated on flexible PI substrate in this work. The performance of IGZO TFTs with different plasma species and treatment time are investigated and compared. The experiment results show that the plasma species and treatment time play an important role in the threshold voltage, field-effect mobility, Ion/Ioff ratio, sub-threshold swing (SS) and bias stress stability of the devices. The TFT with a 10 seconds NH3 plasma treatment shows the best performance; specifically, threshold voltage of 0.34 V, field-effect mobility of 15.97 cm2/Vs, Ion/Ioff ratio of 6.33×107, and sub-threshold swing of 0.36 V/dec. The proposed flexible IGZO-TFTs in this paper can be used as driving devices in the next-generation flexible displays.