A fuel tank-mounting structure for a portable power working machine, which has an internal combustion engine as a power source and a fuel tank received in a tank housing portion defined in a two-piece cover housing of the power working machine, includes cushioning members supporting the fuel tank in the tank housing portion of the two-piece cover housing. The cushioning members are interposed in spaces between the tank housing portion and all faces of the fuel tank, namely, the front and rear end faces, the right and left side faces, and the top and bottom faces of the fuel tank. The cushioning members prevent the vibration of the internal combustion engine from being easily transmitted to the fuel tank, thereby avoiding the generation of bubbles in the fuel as well as damage to the fuel tank, and also reduce the manufacturing cost of the fuel tank-mounting structure.