The aim of this paper is to make a characterization of hot rolled, round steel bar of unknown composition that we could rate the quality of material and possibly its possible application. The work consists of theoretical and experimental part. In the theoretical part, the analysis of chemical composition, qualitative and quantitative analysis of the microstructure and the methods for determining the mechanical properties of materials, are described. The experimental part presents the results of tests obtained by analyzing the chemical composition, qualitative and quantitative analysis of the microstructure, static tensile testing, measuring fracture tendency to Charpy-ement piston and measuring the hardness by Vickers. Results of the analysis of the chemical composition showed that analyzed material in its composition is closest to 42CrMo4 steel or to low alloyed Cr-Mo steel for quenching and tempering. The microstructure of analyzed steel is fully martensitic, and the size of crystal grains is about seven. The measured values for tensile strength and elongation are within the limits for the 42CrMo4 steel in tempered condition which is provided by the standard EN 10083-3: 2006. The values of other mechanical properties do not satisfy the regulations of the standard. The specified standard prescribes the technical conditions of delivery for low alloyed steels for quenching and tempering. Considering that the chemical composition is within the prescribed limits, it is assumed that the poor mechanical properties are results of improper heat treatment of steel.