The effects of neutron irradiation on the unnotched straincycled fatigue properties of pressure vessel steels were evaluated. Two heats of Type A302B low alloy steel and one heat of Type A212B carbon silicon steel were investigated. Results from room temperature tests on nonirradiated material were compared with those from material irradiated up to 6 × 10¹⁹ nvt > 1 Mev (1.16 x 10²³ Dt). The fatigue notch sensitivities of these materials were also investigated using V-notch specimens with 2 and 10-mil root radii. Results were compared with theoretical predictions based on the δ theory of fatigue notch sensitivity.