An end surface structure of a heat pipe having a large gauge used to be contacted with a heat source for dissipation is provided. The heat pipe includes a pipe member, a lid, a base and a wick structure. The hollow tubular pipe member has two opposing open ends. The lid is closely covered on one open end. The base provides an interlocking member including a flange fitly embracing the pipe member to receive the other open end of the pipe member thereinside. The wick structure is attached on an inner wall of the pipe member and an inside surface of the base. Moreover, the thickness of the flange is not larger than the thickness of pipe member at the open end received in the interlocking member. When the base is fitted with the pipe member at the open end, a welding process is performed to permanently connect them together. In the welding process, the flange of the interlocking member is enforced to be liquefied first and is liquefied more than the pipe member at the open end. As such, it can ensure that the pipe member is prevented from being damaged during the welding process.