This paper proposes a simple but powerful approach to determine the constitutive equation of tempered martensitic steels from micro-hardness tests. Finite element simulations were used to investigate the plastic flow in the contact region between the tip and the specimen. The simulations were validated by experimental tests carried out on the tempered martensitic steel Eurofer97 using a simple constitutive Ludwik-type equation. A series of simulations using different constitutive behaviors representative of possible irradiation-induced changes were run. In all cases, a pile-up of material against the indenter tip was observed that is strongly dependent on the constitutive law. Considering the real contact height of the indenter with the material, it was shown that the hardness scales with an averaged value of the flow stress over 30% of plastic strain. In addition, the parameters of Ludwik equation were shown to be determinable from the two experimental quantities that are the hardness and the ratio between the contact height and the penetration depth of the tip. (C) 2013 Elsevier B.V. All rights reserved.