LTPS TFTs with MONOS structure was fabricated to investigate the feasibility of suppressing threshold voltage variations between TFTs. By applying high positive and negative gate bias voltages, threshold voltage could be tuned both positively and negatively by injecting charges to charge trap layer, respectively. Even after tuning, stability of threshold voltage against positive gate bias at the actual circuit operation level was not degraded compared to the case of as-fabricated TFT.