The measurement of range of motion during medical examination is a basic procedure in diagnosis and treatment evaluation. However, accurate measurement requires experience, and it is difficult for patients to measure range of motion by themselves. If patients can easily and accurately measure the range by their own, it will encourage patients to continue rehabilitation and will be useful for helping patients to determine when they should see a doctor. We developed a wrist flexion and extension measurement system using images captured by a smart phone so that patients can measure range of motion without expert knowledge or experience. First, the system calculates a middle line for the forearm and an outside line for the dorsal hand based on user inputs. Afterward, the angle formed by these lines is calculated. The experimental results show that the average absolute errors are 4.82° for flexion and 8.74° for extension.