We have fabricated a LDD structured polysilicon thin film transistor with low leakge current and the optimized LDD length has been obtained. The device performance is improved is improved by hydrogen passivation process. The on.off current ratio of poly0Si TFT s with $0.5{mu}m$ and $1.0{mu}m$ LDD length is much higher than that of conventional structured device due to the decrease of leakege current. The optimized LDD length may be $0.5{mu}$ from the experimental data such as on/off current ratio, threshold voltage and hydrogenation effect.