The effects of impurity iron content on characteristics of sintered reaction-bonded silicon nitrides were examined by adding iron powder to a high purity raw Si powder. Powder compacts of the raw Si powder doped with 2 mol% Y2O3 and 5 mol% MgSiN2 as sintering additives and Fe as impurity (0 mass%, 0.1 mass%, 1.0 mass% and 5.0 mass%) were nitrided at 1400°C for 8 h under a N2 pressure of 0.1 MPa, followed by post-sintering at 1900°C for 6 h under a N2 pressure of 0.9 MPa. All the SRBSN (Sintered Reaction-Bonded Silicon Nitride) specimens had about the same 4-point bending strength of 730–770 MPa. The fracture toughness of the specimens was gradually decreased with increasing Fe additive amount due to the inhibition of development of rodlike β-Si3N4 grains by SiFex particles formed during nitridation process. The thermal conductivity was also decreased with an increase in Fe amount. It seems that the increasing oxygen in grain-boundary phase caused by the oxidation of Fe during milling resulted in the increase in lattice oxygen of β-Si3N4 grains, which caused phonon scattering and thereby decreased thermal conductivity of β-Si3N4. There was little change in the dielectric breakdown strength of the specimens: 24, 22, 22, and 21 kV/mm for the specimens without Fe, and with 0.1 mass%, 1.0 mass% and 5.0 mass% Fe, respectively. The surface resistivity of the specimens with 0 mass%, 0.1 mass% and 1.0 mass% Fe was in the range of 1013 Ω, but the specimen with 5 mass% Fe was about one order lower than the others.