Mechanical properties of steels are a function of their chemical composition and microstructure resulting from various physical processes going on during steel processing and heat treatment. Heat treatment, as a very important step in the development of production processes, is nowadays a well-controlled and mainly automated process in the phase of heating and maintaining workpieces at the austenitizing temperature; the cooling process, i.e. quenching, which predominantly affects final properties of a product, however, is itself affected by a number of physical and technological variables. To measure the residual stresses, the relaxation method was used, involving gradual mechanical removal of the hardened layer in which the deformation of the specimen was measured by resistance strain gauges. On the basis of the measured deformation at a specific depth of material removal, it is possible to calculate the size or variation of residual stresses depending on the depth of the hardened layer. Numerous residual stresses measurements provided very important information on various steels when using different quenching agents.