In graduation thesis titled „Characterization of microstructure of low-alloy steel“ the microstructure of two low-alloy steels is analyzed. Thesis consists of two parts – theoretical an experimental. In the theoretical part, the influence of microstructure and chemical composition on the properties of metallic materials and the properties of steel is analyzed. The procedures for characterization of the microstructure, the principle of operation and characteristics of the optical and electron microscope and the preparation of samples for the same are described. In the experimental part of the paper, the microstructure of the steel for cementing EN C15E and the steel for improving EN 51CrV4 was characterized by optical microscopy. Both steels have a ferritic-perlite microstructure. In the case of steel EN C15E, the individual phases differ better and a point analysis was performed manually to obtain a perlite percentage of 22 %. The microstructure of EN 51CrV4 steel is finer and it is assumed that the reason for this is the alloying of steel with vanadium which grinds the primary austenitic grain. Using a structural diagram based on the taken mean value of 0.51% C of carbon for EN 51CrV4 steel, a perlite content of 64% and a ferrite content of 36% were obtained.