In the 2Si-3Ni steel intercritically treated in the range of 720 ‡C to 790 ‡C, the fracture behavior under the impact testing has been analyzed and the post-tempering effect has also been investigated. The transgranular fracture occurred in the specimens treated below and at 730 ‡C (SN73 specimen) in relatively low intercritical temperature range, but the intergranular fracture occurred in the specimens treated at 750 ‡C and 770 ‡C (SN75 and SN77 specimens) in relatively high intercritical temperature range. In the SN73 specimen, there was little coarse martensite at the prior austenite grain boundaries, whereas there was continuous, coarse martensite at those boundaries in the SN75 and SN77 specimens. The fracture behavior was mainly discussed in terms of the microstructural differences. In addition, no or a little increase in impact toughness, in spite of great decreases in hardness, in the SN75 and SN77 specimens tempered at 600 ‡C is correlated with the easy occurrence of intergranular fracture, which is caused by the carbide aggregates formed in the continuous, coarse martensite at the grain boundaries.