A neutron irradiation for thermal neutron flux density of 5 × 10¹¹/cm²s and irradiation time of 10 hr was applied to hydrogenated amorphous silicon (a-Si:H). The density of ³¹P produced by the neutron transmutation doping (NTD) was estimated to be 10¹²/cm³. Dark and photo conductivities were measured before and after NTD in parallel with ESR measurements. The annealing behavior of the irradiation damage was studied up to the growth temperature of a-Si:H. Dark conductivity of the well-annealed NTD a-Si:H shows an activation type conduction above room temperature with activation energy of 0.69 eV.