Without sacrificing the on-current in the transfer characteristics, we have successfully reduced the off-current part by the optimal N2O plasma treatment to improve the on-off-current ratio, in n-type titanium oxide (TiOx) active-channel thin-film transistors. While the high-power (275 W) N2O plasma treatment oxidizes the whole TiOx channel and results in the reduction of both on- and off-current, the optimized low-power (150 W) process makes the selective oxidation of the top portion in the channel and reduces only the off-current significantly. Increase in on-off ratio by almost five orders of magnitude is achieved without change in on-current by using the presented method.