This research aimed to improve the mechanical properties of high-Cr steels by developing new thermomechanical processes for application to advanced fast reactors. A simultaneous improvement of fracture toughness and strength was specifically targeted in the design of thermomechanical processing for various 9Cr and 12Cr steels. For the structural materials of fusion and fast reactors, the high-chromium steels with fine ferritic/martensitic (FM) structures are favored because of their high creep strength and excellent resistance to high-temperature radiation damage such as void swelling. After the traditional thermomechanical treatment consisting of normalizing, water quenching, and tempering, however, the high-Cr steels show limited mechanical properties at high temperatures, e.g., significant decrease of strength above ~450°C. Since the high-Cr steels are tempered at high temperatures (typically around 750°C), significant coarsening of the lath structure and carbides and annihilation of dislocations occur in the final thermomechanical treatment. It is believed that the high-temperature tempering can maximize the ductility of the FM steels, but their high-temperature strength is reduced and often with high-temperature fracture toughness. It is also likely that such a fully-tempered structure will demonstrate a lowered radiation resistance when compared to the steels with finer structures. This research was planned to explore new processing routes that can produce ultrafine microstructures and thus can improve the high-temperature mechanical properties of the high-Cr steels. In this research, three 9Cr steels and two 12Cr steels were selected and a series of new thermomechanical process treatments (TMTs) were designed and applied to the five steels to find the TMT routes that will result in increase of both strength and fracture toughness. It was demonstrated for HT-9 steels that some tailored TMTs increased or maintained the fracture toughness of the high-Cr steels while the treatments could significantly increase their strength to the level comparable to those of nanostructured ferritic alloys. This report compares the fracture toughness data of both 9Cr and 12Cr steels after various thermomechanical treatments with the reference data of similar steels. A two-step tempering route is selected for application to the high-Cr steels for reactor core structures.