Effects of fast neutron irradiation and post-annealing on magnetic properties of Rb 3 C 60 were studied through the dc magnetization measurement. Rb 3 C 60 powder samples were prepared in an evacuated quartz glass tube, and the temperature and the magnetic field dependences of dc magnetization were measured before and after irradiation and after post-annealing. The neutron fluences were 1.0, 1.8 and 3.3×10 16 n/cm 2 , and the post-annealing was made at a temperature of 473K for 3h. Magnetic hysteresis of the samples irradiated at the fluence of 1.8 and 3.3×10 16 n/cm 2 disappeared, and the hysteresis curves hardly changed at the fluence of 1.0×10 16 n/cm 2 . As for the post-annealing effect, the hysteresis curves of the sample irradiated at the fluence of 1.8×10 16 n/cm 2 were completely recovered after annealing, while those of the other samples, which had a magnetic background before irradiation, were not recovered. In this study, it was found that the loss of superconductivity in Rb 3 C 60 powder is observed when the neutron irradiation fluence exceeds 1.0×10 16 n/cm 2 , and the lost superconductivity is completely recovered by the post-annealing at 473K for 3h.