Summary: A hardening stress strain law is determined for the AISI M2 cold forging tool steel. The experimental part is based on the Brinell indentation test. The numerical part consists in the simulation of the Brinell indentation test under increasing load cases. For each load case, parameters of the behaviour law are adjusted in order to make the numerical diameter of the Brinell test converging towards the experimental one. Results are compared with a rigid perfectly plastic behaviour law usually used for this kind of steel. Then, a deformable sphere is used and the results are compared with a rigid one. It is shown that a hardening behaviour law is more accurate than a rigid perfectly plastic one for the study of wear and tools damage since it is more representative of the reality.