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A large gas turbine disk of IN718 alloy was produced by a process that gives a very fine microstructure. The specimens, directly cut from the component, have been tested in the low cycle fatigue (LCF) and in the fatigue crack propagation (FCP) regimes at 650°C in air and in vacuum. In air a low strain rate or a low test frequency give a reduction of fatigue life and fatigue crack propagation rate (FCPR). When LCF tests are performed in vacuum, fatigue life increases and the influence of strain rate is sensibly reduced. In vacuum FCPR does not depend on test frequency. The evaluation of fatigue damage mechanisms has been performed analysing the fracture surfaces by scanning electron microscopy. At least the life prediction fatigue parameters according to Coffin-Manson and Paris relationships have been calculated and discussed.