The magnetic flux loss induced in sintered Nd-Fe-B permanent magnets by proton and a irradiation has been measured. The irradiations have been made with 14 -20 MeV protons and with 56 MeV a particles. The dependence of the magnetic flux loss on the temperature, external magnetic field, and on the particle energy has been studied. An earlier theoretical model of the magnetic damage is refined to take into account the effects of the external magnetic field as well as the shape of the magnet sample. The good agreement between the experimental and theoretical results supports our theoretical model according to which the local heating induced by particle irradiation causes the magnetic flux loss.