Abstract: Microstructure and room temperature impact toughness of HR3C steel aged at 600 ℃ for different times were investigated by means of optical microscopy, scanning electron microscopy, energy dispersive spectrometer testing, X-ray diffractometer testing and impact testing. The results show that the microstructure of aged HR3C steel consisted of austenite and precipitated phases mainly including Nb(C,N) and M23C6. With the increase of aging time, the amounts of precipitated phases increased continuously, and M23C6 precipitated phase precipitated along grain boundaries, gathered and grew up into the bulk, finally connected to each other. At the beginning of aging, impact absorbed energy of the steel decreased obviously and the dimples was priority on fracture. After aging for 200 h, the decrease of impact absorbed energy became slow and the numbers of intergranular cracks on fracture increased. The intergranular fracture was main after aging for 500 h.