The main part of this master’s thesis is to evaluate the hardening depth of the machine part MO.20- DP.28. The material from which the machine part is made is 16MnCr5 steel. The method used to estimate the depth of hardening is the Jominy test. In the initial part of the paper, the martensitic transformation in steel as a starting point for heat treatment of hardening and its properties is explained. The TTT diagrams show the dynamics of phase transformations in steel, and the difference between the isothermal TTT diagram and the TTT diagram for continuous cooling. In addition to defining the quenching process, the Rockwell hardness test is described, as well as hardening and hardenability as properties of steel on which the production of microstructure during steel hardening depends. Jominy’s method of hardenability testing is described in detail, as is the determination of the hardness flow curve using Crafts-Lamont diagrams. In the practical part of the thesis, the Jominy experiment was started by presenting the design of the machine part MO.20-DP.28 and presenting the basic information about 16MnCr5 steel. All the necessary equipment is described and the process of quenching of the sample and hardness testing are shown. The obtained values were used to make a Jominy curve, and using Crafts-Lamont diagrams, a hardness flow curve was created. It was concluded that with the quenching intensity for oil H = 0.4, the machine part MO.20-DP.28 made of 16MnCr5 steel its not sufficiently hardened for a given diameter.