This paper proposes an optimal mathematical model to represent the surface of hand wrist. First, three\|dimensional photogrammetry is used to take photos of the touched surface of a hand wrist. Then, a system of digital photogrammetry is taken to capture the data from the photos. Finally, the optimal mathematical model is presented to represent the touched surface of the hand wrist. The result may not only show the optimal coincidence between two wrist surfaces to the surgical of hand surgery, but also benefit the make of artificial joint.