Abstract: 20Cr32Ni1Nb steel was aged at 950 ℃ for different time (200-5 000 h), and the effects of aging time on microstructure and mechanical properties of the steel were studied. The results show that with the increase of aging time, the lamellar structure of eutectic carbides on grain boundaries of the 20Cr32Ni1Nb steel disappeared, and then the structure changed from short rod shape to granular shape gradually. The secondary carbides precipitated from the austenite matrix and diffusely distributed, and the number of secondary carbides was the largest after aging for 500 h. G phase precipitated from 20Cr32Ni1Nb steel when the aging time was 5 000 h. With the increase of aging time, the tensile strength and yield strength of 20Cr32Ni1Nb steel both increased first and then decreased, the elongation decreased, and the impact toughness decreased first, then increased and then decreased. The impact toughness was 40 J·cm^-2 after aging for 5 000 h.