A buried word line structure, including a first isolation structure, a buried word line, a first barrier layer, a second barrier layer, a channel layer, and a second isolation structure, is provided. The first isolation structure is disposed in the substrate and has a trench. The buried word line is disposed on a bottom surface of the trench. The first barrier layer is disposed between the buried word line and a sidewall and the bottom surface of the trench. The second barrier layer covers a top surface of the buried word line and includes a main portion and an extension portion. The main portion is located on the buried word line, and the extension portion extends upward from periphery of the main portion. The channel layer is disposed on the first barrier layer and the second barrier layer. The second isolation structure is disposed on the channel layer.