The scope of this doctoral thesis has been the investigation of mechanisms and kinetics of microstructure transformation as well as the study of mechanical properties, with the objective of a more accurate defining of physical phenomena during steel quenching. Increasing technical requirements, relating to the quality of mechanical properties of the engineering components, imply a deep understanding of relations among microstructure, chemical composition and mechanical properties. Although the process of steel quenching is simple to apply, it is one of the physically most complicated engineering processes, which involves many interacting processes: physical processes of microstructure transformation, processes of heat exchange, transfer and heat conduction, processes of generation of deformation and residual stresses, and processes of crack formation and its growth. Based on theoretical investigations, methods for determination of kinetics parameters of isothermal austenite decomposition into ferrite, pearlite and bainite have been proposed, as well as equations of austenite decomposition kinetics. Furthermore, on the basis of the chemical composition of hypoeutectioid steels, equations for the estimation of thermodynamic constants of austenite decomposition into ferrite, pearlite and bainite have been put forward. Equations for the evaluation of the microstructure constituents` hardness have been also presented. A proper algorithm developed in order to predict the austenite decomposition kinetics as well as the microstructure constituents` hardness has been implemented in the 3‐D computer program for a 3‐D simulation of the specimen`s cooling, whereby the 3‐D simulation of the austenite decomposition during steel quenching is enabled. Low‐alloy steel for tempering 42CrMo4 (DIN) has been applied for the verification of results obtained by the computer simulation. The results of the computer simulation show that developed mathematical models of microstructure transformations and hardness can be efficiently used for the prediction of austenite decomposition during steel quenching.