An open wrench includes a body having a mounting hole defined therethrough. Two first faces and two second faces are respective defined in the inside of the mounting hole. The two first faces and the two second faces are located along four consecutive sides of an imaginary hexagonal body. Each first face has at least two teeth defined therein. Five curved faces are defined in the inside of the mounting hole and located corresponding to five corners of the imaginary hexagonal body. Each of the first and second faces has a contact face and each contact face is connected between two adjacent curved faces. The length of each contact face is 0.5-0.65 of the length of each side of the imaginary hexagonal body.