The precipitation kinetics of σ phase in commercial HR3C heat-resistant steel during aging at 650-800 ℃ was studied in the paper.Through morphology,composition and structural analyzing on the second phase in the HR3C steel,it was confirmed that the precipitations after aging were mainly NbCrN,M23C6 and FeCr type σ phase.The time-dependent mass change of the three precipitated phases showed that the linearly increased σ phase after aging at 750 ℃-2000 h was transformed from NbCrN phase or M23C6 phase.According to the calculation on the volume fraction of electrolytically dyed σ phase,the time-temperature transformation (TTT) curve for σ phase at 1 vol％ in two kinds of commercial HR3C steel (different in grain size) was obtained and analyzed.The nose of the TTT curve was located at around 750 ℃ for the two kinds of HR3C steel,and the larger grain size HR3C steel displayed a inhibit effect on the precipitation of σ phase.The impact energy of the HR3C steel after aging at 700 ℃ decreased obviously with the fracture mechanism changing from ductile fracture to intergranular brittle fracture,which was considered to be related to the density of σ-brittle phase after aging.