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We have analyzed the mechanism of off-leakage current in an lightly doped drain (LDD) poly-Si thin-film transistor by investigating the activation energy Ea. It is found that Ea decreases as the gate and drain voltages increase. We have also discussed the mechanism using a device simulator. It is found that a hole channel is lightly formed in the LDD region, and a pseudo p-n junction appears at the junction between the LDD and drain regions. The aforementioned experimental behavior of Ea is because the electric field increases there. These results suggest that the off-leakage current is caused by the phonon-assisted tunneling with Poole-Frenkel effect at the junction between the LDD and drain regions.