The apparatus of measuring a workpiece according to the present disclosure includes a base, a measuring member, an upper arm member, a lower arm member, a first pin and a second pin, wherein the upper arm member includes an upper rotation frame and the lower arm member includes a lower rotation frame. When the first pin is moved to disconnect the upper rotation frame from the lower rotation frame, the upper rotation frame is rotatable. When the second pin is moved to lock the lower rotation frame to the measuring member, the lower rotation frame is free of rotation. When the first pin is moved to connect the upper rotation frame and the lower rotation frame with each other and when the second pin is moved to unlock the lower rotation frame from the measuring member, the lower rotation frame is rotatable with the upper rotation frame.