In this master's thesis the estimation of hardening depth of workpiece HT.22-DP.68 was approximated. In the first part, the concept of steel is discussed and the influence of application of impurities, non-metallic inclusions and alloying elements are explained. In the middle part of this thesis, the theory of steel heat treatment, phase transformation of steel due to steel treatment as well as quenching, hardening and hardenability are explained which contribute to better understanding of experimental part. Afterwards, methods of estimation of hardening depth (Jominy method, Just method, Grossmann method) were listed. The most common method of the estimation of hardening depth is Jominy method on which is experimental part based. The Jominy method is based on a quenching and hardness testing of the sample. The final part refers to the hardness testing and estimation of hardening depth of workpiece HT.22-DP.68 by application of the Jominy experiment. In addition, the plant of performing the Jominy experiment is described, as well as all the all necessary equpment used in performing the same. Based on a measured results, a Jominy curve was constructed. Furthermore, using the resulting curve and Craft-Lamonts graph, the hardenability values of the workpiece HT.22-DP.68 for the quenching in water (H = 1) were obtained. Based on the obtained values, the U-curve was draw which represents flow of hardness in the cross section of the workpiece HT.22-DP.68. From everything quoted it can be concluded that the workpiece HT.22-DP.68 is completely hardened.