The paper presents the results of research on the intergranular corrosion resistance of austenitic steel HR3C after long-term isothermal ageing at the temperature of 700 °C and time up to 40 000 h. During ageing the precipitation of M23C6 carbides as well as sigma and Z phases was observed in the steel microstructure. The susceptibility of the steel to intergranular corrosion was assessed on the basis of the Degree of Sensitization (DOS) ratio. It has been proved that in the initial ageing period of up to around 1000 h, the HR3C steel shows increased susceptibility to sensitization, connected with the precipitation of M23C6 carbides (DOS = 68%). As the time of ageing increased, the process of desensitization proceeded, leading to a decrease in the value of DOS to around 0.20% after 40 000 h. It has been concluded that the precipitation of sigma phase does not cause sensitization of the steel to intergranular corrosion. The obtained results indicate that long-term ageing of HR3C steel at the elevated temperature does not have to lead to the growth of the susceptibility to intergranular corrosion, and can promote its desensitization.