A method of forming a TFT structure is performed on a glass substrate. A first metal layer deposited on the glass substrate is patterned with a first mask to form a gate line and a gate electrode. Next, a gate insulating layer, a first semiconductor layer and an etch-stop layer are successively formed, and backside exposure patterns the etch-stop layer. Thus, the remaining part of the etch-stop layer is disposed over the gate electrode and the gate line. Next, a second semiconductor layer and a second metal layer are successively formed, and then the second metal layer is patterned with a second mask to form a data line perpendicular to the gate line. Thereafter, the second semiconductor layer and the first semiconductor layer not covered by the second metal layer are removed. Next, a first protection layer formed on the exposed surface of the glass substrate is patterned with a third mask to form a first opening and a second opening, wherein the first opening is over the gate electrode and the second opening is over a predetermined drain electrode. Next, a conductive layer and a photoresist layer successively formed on the exposed surface of the glass substrate are patterned with a fourth mask to form a pattern of a predetermined pixel electrode. Finally, after removing the second metal layer and the second semiconductor layer underlying the first opening to expose the etch-stop layer, a second protection layer is formed on the first protection layer to fill the first opening.