This article describes the studies of the mechanical properties of the martensitic structure of C45 steel, obtained as a result of heat treatment by a corresponding cooling method. This steel was subjected to high tempering, within the temperature range of 500÷700o C (every 50o C) and for various exposure times, from 15 minutes to 23 hours. Moreover, martensitic steel was subjected to non-standard tests by quenching at a temperature of 850 oC for 20 minutes and then tempering it for 1 hour, within the temperature range of 50÷800o C (50o C). The resulting steel structure variants were subjected to strength tests, that is, to tensile and hardness tests and also to tests on the micro-structure. The results of these studies are presented and discussed, in detail, in terms of their practical application.