Abstract: HR3C steels with three grain morphologies (fine, coarse and elongated) were prepared by hot forging and solution treatment. The microstructure, precipitation morphology, hardness and impact toughness were investigated after aging at 700℃ for 500-2 000 h, and the effect of grain morphology on aging brittleness was discussed. The results show that HR3C steel with coarse grains had uniform grain size distribution, and M₂₃C₆ carbides continuously precipitated along grain boundaries. The steel had low hardness and impact toughness, and exhibited brittle fracture after aging. HR3C steel with fine grains had uniform grain size distribution, M₂₃C₆ carbides were distributed in chains at grain boundaries. The steel had higher hardness and impact toughness, and exhibited a mixed fracture of brittleness and ductility after aging. HR3C steel with elongated grains had an uneven grain size distribution, and M₂₃C₆ carbides had a discontinuous distribution grain boundaries. The steel had high hardness and impact toughness, and exhibited ductile fracture after aging.