This paper presents the results of a steel magnetic property study using a non-destructive method of determination of the mechanical properties of products made from structural steels 20, 09Mn2Si, and 25CrMoV. The influence of tempering temperature on the change of magnetic properties was analyzed and a statistical analysis of the input parameter significance, including coercive force, residual magnetic induction, and maximum magnetic permeability, was carried out. As a result, it was found that the greatest sensitivity for steels at tempering temperatures up to 400 °C is the coercive force and, above 300 °C, the residual magnetic induction. At the same time, the maximum magnetic permeability provides a correlation over the whole range of tempering temperatures, justifying the use of magnetic properties in the determination of mechanical properties.