Fatigue life and crack growth rate comparisons have been made of irradiated and unirradiated specimens of Type 348 stainless steel as a function of total strain amplitude. Notched specimens were cycled at resonance in reverse bending in dry air at constant amplitude, and crack growth rates were determined from changes in resonant frequency of vibration. Neutron irradiation to a fluence of 1.5 × 10²¹ n/cm² > 0.1 MeV produces approximately an order of magnitude increase in fatigue life at a total strain of 0.20 percent. At lower strains irradiated fatigue life appears to approach an endurance limit and the increase in life becomes very large. The effects on life are attributed to decreased crack growth rates in the irradiated material. Thus, the results in dry air indicate that the fatigue performance at 550 F (288 C) of thin-section reactor components of Type 348 stainless steel should not be reduced by irradiation characteristic of pressurized water reactors. The influence of water vapor on the fatigue characteristics of irradiated Type 348 stainless steel is being investigated in a separate study.