Low activation austenitic steels of Fe-Cr-Mn-Ni-C-N were designed using the d-electron concept, which was devised on the basis of the molecular orbital calculation of transition-metal based alloys. In this design two calculated parameters are mainly utilized. The one is the d-orbital energy level (Md) of alloying transition elements, and the other is the bond order (Bo) that is a measure of the covalent bond strength between atoms. Alloying effects on the microstructures and physical properties of steels are well understood using these parameters. The phase stability of various austenitic steels containing chromium, manganese, nickel, carbon, and nitrogen were examined experimentally. The results were interpreted using these parameters, and a new phase stability index diagram was proposed for the austenitic steels. In this diagram, the γ/γ + αM and the γ/γ + α(δ) phase boundaries are well represented for solution-treated alloys, and also the γ/γ + σ phase boundary is denned for aged alloys. Here, γ is the face centered cubic (fcc) matrix phase, and αM, α(δ) and σ are the martensite, ferrite, and sigma phases, respectively. Computer-aided design was performed so that the steel could contain the least amount of high activation elements such as nickel and nitrogen, and several alloys were selected as potential materials for fusion reactor.