The purpose of hardening is the increase the hardness of the steel by creating an imbalanced structure. High hardness can be achieved by the formation of a martensitic structure. In some specific cases, we try to obtain a bainitic structure when hardening. In real hardening processes, a martensitic structure is usually achieved on the surface of the body, producing a mixture of martensite, bainite and other types of microstructures in the core of the body. The contribution deals with the influence of anisothermal hardening on selected mechanical properties of EN ISO S235JR (G2) steel. The observed values are the HBW hardness, Rm strength and hardness A. At the hardness measurement locations, the structural steel compositions are metallographically evaluated in the surface of hardened and non-hardened samples that are mutually compared. Subsequently, the influence of hardening on the examined mechanical properties and dependencies descibing the correlation of the structure and hardness of the hardened steels is deduced.