The engineering and economical aspects of the optimization of steel shaft quenching and tempering were investigated. The mathematical model and method of computer simulation for the prediction of fatigue properties of quenched and tempered steel were developed. Computer simulation of the fatigue properties of quenched and tempered steel was applied in the optimization of quenching and tempering of steel shafts. The proper heat treatment process was accepted based on economical analysis. Fatigue properties of quenched and tempered steel were predicted based on micro-structure composition and yield strength. Microstructure composition and yield strength were predicted based on as-quenched hardness. The distribution of as-quenched hardness in the workpiece was predicted through computer simulation of steel quenching using a finite volume method. The as-quenched hardness was estimated based on time of cooling and on Jominy test results. It was taken into account that the mechanical properties of quenched steel directly depend on the hardness, degree of hardening, and microstructural constituents. Using a numerical simulation of microstructure and mechanical properties, it was found out that the investigated shaft has the best fatigue properties in cases when the shaft was machined or formed on proper geometry before proper quenching and tempering. Through economical analysis of the investigated shaft manufacturing, it was found out that the most suitable shaft manufacture process is to manufacture the shaft from the quenched and tempered bar. But in this case, heterogeneous microstructures of ferrite, perlite, bainite, and martensite with very low fatigue limits could appear at some critical locations.