The Inconel alloy 718 is an iron-nickel based superalloy with a working temperature up to 650 °C. Presented phases such as γ'' (Ni₃Nb), γ' (Ni₃Al), and δ (delta – Ni₃Nb) are responsible for the alloy's unique properties. The δ – delta phase is profitable when situated at grain boundaries in small quantities due to increasing fatigue life. However, at temperatures close to 650 °C the γ'' transforms to δ – delta and causes a decrease in fatigue life. Heat-treatment (800°C/ for 72 hours) and its influence on fatigue life are discussed in this paper. Fatigue tests were carried out at room temperature. After the tests we plotted the S-N curves for both stages. SEM (Scanning Electron Microscopy) fractography was carried out as well.