A positioning system for positioning a body part of a patient for radiotherapy includes a positioning device and a radiation treatment system. The positioning device includes a photosensor disposed on an outer surface of the body part and configured to generate a sensing signal upon sensing a positioning beam. The processing unit is operable in a recording mode and a comparison mode. In the recording mode, the processing unit receives the sensing signal of the photosensor and generates reference positioning information according to the sensing signal. In the comparison mode, the processing unit receives the sensing signal of the photosensor and generates comparison positioning information according to the sensing signal. The processor compares the reference positioning information to the comparison positioning information and outputs a positioning result according to a result of comparison.
