Hydrogen related defect states generated in n-FZ Si(H2) single crystal after neutron irradiation have been studied by IR absorption. We observed some new absorption peaks that have not yet been reported. These peaks appear at 1992cm-1 and 1857 cm-1 in non-irradiated and irradiated specimens respectively. All absorption peaks arising from hydrogen and hydrogen related defects of n-FZ Si(H2) before and after neutron irradiation have been both discussed and assigned. And the 2150cm-1 absorption peak is attributed to the hydrogen donor.