© 2020 The Authors Spinodal decomposition in thermally aged Fe–Cr alloys leads to significant hardening, which is the direct cause of the so-called 475∘C-embrittlement. To illustrate how spinodal decomposition induces hardening by atomistic interactions, we conducted a series of numerical simulations as well as reference experiments. The numerical results indicated that the hardness scales linearly with the short-range order (SRO) parameter, while the experimental result reproduced this relationship within statistical error. Both seemingly suggest that neighboring Cr–Cr atomic pairs essentially cause hardening, because SRO is by definition uniquely dependent on the appearance probability of such pairs. A further numerical investigation supported this notion, as it suggests that the dominant cause of hardening is the pinning effect of dislocations passing over such Cr–Cr pairs. Moreover, this finding has a practical merit, that is, the SRO parameter can serve as a good index of hardening of steels, which is critical for evaluating their lifetime.