In modern steelmaking, carburizing steels are often used for production of automotive components with elevated levels of toughness and fatigue strength. This study is focused on the link between the characteristics of non-metallic inclusions in the steel and the machinability of the given steel grades. For this purpose, inclusion characteristics (such as composition, number, size, morphology etc.) in steel samples were determined by common two-dimensional (2D) observations of inclusions on polished metal surface of samples as well as by three-dimensional (3D) investigations of inclusions after electrolytic extraction from steel samples. The inclusion characteristics were investigated in the three types of 20NiCrMo carburizing steel grades: Steel A - reference steel (410ppm S) having a common level of mechanical properties and machinability, Steel B - steel grade (having 340ppm S) with an improved machinability due to the Ca-treatment and modification of MnS inclusions and Steel C - clean steel grade (40ppm S) having a small amount of inclusions and a higher fatigue strength. It was found that the number of inclusions in Steel B is smaller than that in Steel A and that the inclusion composition is different. At the same time, the estimated machinability (tool life) of the Ca-treated steel was superior to that of the reference steel. However, the significant decrease of the number of inclusions in the clean steel (Steel C) resulted in a lower machinability in comparison to the reference steel grade.