Differences of high energy-ion induced microstructure of bcc δ-ferrite and fcc austenite matrix, and the effects of δ-ferrite on the Vickers micro-hardness (Hv) after irradiation were investigated for Type 304 stainless steel weldments containing two different δ-ferrite contents: ferrite number (FN) 5.5 and 8.5, respectively. Specimens were irradiated to 1.5 dpa with 8 MeV Fe⁺⁴ ions using a Tandem Vande-Graff accelerator (flux: 4.3×10¹⁰ ion/cm²·s, fluence: 0.83×10¹⁵ ion/cm²) at below 60°C. Calculations TRIM 95 showed that a peak damage appeared at 1.5 \\micron in depth with 0.7 \\micron full width at half maximum (FWHM). These results on irradiation-induced defects (IIDs) distribution were confirmed by TEM. Clear differences for the size and number density of IIDs as black dots (size: 5–10 nm) and loops were observed in both the austenitic matrix and δ-ferrite, where the size of IIDs was far larger in the fcc matrix than the bcc δ-ferrite. Hv test results showed that the irradiation hardening of δ-ferrite was about 1.5 times larger than the austenitic matrix. From microstructural observation the increase of the higher Vickers micro-hardness was explained.