Processes of aging of maraging steel 03N18K3M3T from commercial heats are investigated. Dilatometric, x-ray diffraction, and fractographic studies and tests for corrosion strength and delayed fracture (DF) are described. It is shown that after quenching from 820°C and aging at 400°C a DF crack propagates in the studied steel over grain boundaries at a high speed, which causes brittle fracture. The corrosion rate of the steel after such a treatment is also high. X-ray diffraction analysis has shown the presence of a strong and virtually single-component texture in martensite, which is preserved after forging, prior high-temperature quenching, and base quenching from 820°C.