Abstract: A thin film transistor and a manufacturing method thereof are provided. A bottom gate, a gate insulating layer and an amorphous semiconductor layer are formed on a substrate. The amorphous semiconductor layer has an uneven upper surface. A laser annealing process is performed on the amorphous semiconductor layer through the uneven upper layer to transform the amorphous semiconductor layer into a polycrystalline semiconductor layer having a smaller-crystallizing-section and a greater-crystallizing-section. Another gate insulating layer, an upper gate and patterned photoresist layer are formed on the polycrystalline semiconductor layer. Patterns of the upper gate and the bottom gate are defined by the same photo-mask. A source/drain is formed in the polycrystalline semiconductor layer. An etching process with etching selectivity is performed on the upper gate and the patterned photoresist layer to make a length of the upper gate shorter than that of the bottom gate.