LTPS TFTs with MONOS structure was fabricated to investigate the feasibility of suppressing threshold voltage variations between TFTs. By applying relatively high positive and negative gate bias voltages, threshold voltage could be tuned positively and negatively by injecting charges to charge trap layer, respectively. Stability of threshold voltage against positive gate bias at a level close to the actual circuit operation was not degraded compared to the case of as-fabricated TFT. Uniformization of threshold voltage distribution could be achieved in a preliminary test using 16 TFTs by converging threshold voltages with a target value by tuning threshold voltages for each TFT.