The relationship between the addition of minor element and the swelling behavior was investigated using six heats of Fe-15Cr-20Ni austenitic model alloys and a PNC 1520 steel irradiated at 480, 570, 620 and 700°C in the Joyo to doses of 20–56 dpa. Phosphorus and boron suppressed the appearance of swelling for these experimental irradiation conditions. Additions of titanium and niobium were effective for suppression of swelling at low temperature, below 570°C. Radiation -induced precipitation is an important factor for controlling the appearance of swelling. The radiation-produced phases in PNC1520 during neutron irradiation are classified into four groups: (1) Frank loops with Ti aggregation on the habit plane, (2) Rod-type precipitates of Fe2P, (3) Large bulk-type precipitates of M23C6, and (4) Blocky-type precipitates of MC with Ti, M6C, Ni-Laves and TiP. The phosphide precipitates strongly suppressed swelling in the low temperature regime as cavity formation started only after phosphide dissolved during neutron irradiation.