This article mainly presents the application of Geant4 package (Geant4.7.0, 2005), developed by CERN (the Center of European Research of Nucleus), in medical physics and a novel technique, namely, hadrontherapy. The distribution of energy deposition in a water model by different radiation beams was also given. The results show that the distributions for proton beams are completely different from those for other radiation beams. Charged particles like protons exhibit little scattering when they penetrate the matter and give the highest energy deposition near the end of their range just before they reach the resting state. These characteristics permit very precise control of the shape of the energy distribution inside the patient's body.