A method of forming a thin film poly silicon layer includes following steps. Firstly, a substrate is provided. A thin film silicon layer is then formed on the substrate by a silicon thin film deposition process. A heating treatment is then applied to the substrate so as to convert the thin film silicon layer into a thin film poly silicon layer. A method of forming a thin film transistor includes following steps. A first patterning process is performed on the thin film poly silicon layer on the substrate to form a semiconductor pattern. Subsequently, a gate insulation layer, a gate electrode, a source electrode and a drain electrode are formed.