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A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical properties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M23C6carbides, Laves, σ and G phases occurred at the grain boundaries. However, Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound complexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M23C6carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C.