A method of forming a polycrystalline silicon layer includes forming a first amorphous silicon layer and forming a second amorphous silicon layer such that the first amorphous silicon layer and the second amorphous silicon layer have different film qualities from each other, and crystallizing the first amorphous silicon layer and the second amorphous silicon layer using a metal catalyst to form a first polycrystalline silicon layer and a second polycrystalline silicon layer. A thin film transistor includes the polycrystalline silicon layer formed by the method and an organic light emitting device includes the thin film transistor.