The Snoek peak of internal friction of the Fe-0.21wt%-0.2wt%Cu alloy neutron-irradiated at 330–341°C, was measured to detect the resolution of carbon atoms. The resolution temperature of carbon atoms in the neutron-irradiated alloy is about 400°C, but that in the unirradiated alloy is about 200°C. The sources of the resolution of carbon atoms are considered cementite and irradiation defects which trap carbon atoms in the irradiated alloy, but in the unirradiated alloy, interstitial carbon atoms come from cementite. Copper in α-iron delays the resolution of carbon atoms.