Abstract. High-carbon low-alloyed steels are promising materials to increase abrasive wear resistance of machine parts working under severe wear caused by hard particles. These steels being quenched from 900-1000 oC possess mostly retained austenite structure which is capable to transform to martensite in the course of abrasive wear. The isothermal holding is known to serve as very beneficial yet long operation which provides bainite transformation with significant improvement of mechanical properties. The goal of present work was to investigate the changes in microstructure on very beginning of isothermal holding. Samples of high-carbon low-alloyed steel (1,21 % C; 2,56 % Mn; 1,59 % Si mass.) were quenched from 900 оС to obtain in microstructure mostly residual austenite and minor quantity of martensite. Isotermal holdings at 250 oC and time frame from 1 to 12 hrs were made to initiate bainite transformation and investigate the accompanying changes of microstructure. The first investigated microstructure that corresponded to 1 hour holding has revealed distinctive signs of beginning bainite transformation, i.e. long laths of bainitic ferrite which appeared in the vicinity of pre-existed martensite. Additionally another structure was found along with laths of bainitic ferrite after holding during 3 hours and more. The structure appeared as very fine lamellar mixture with interlath space far less than 100 nm. According to known references this structure may be addressed as columnar bainite. «Hardness vs time» dependence of the material in the course of isothermal holding has the minimum after 1 hour holding. At that tempering of martensite is finished and bainite transformation enters its accelerated phase. The future work may be devoted to investigation of developing mechanical properties of 120Mn3Si2 steel and quality of welding joints after isothermal holdings up to 6–7 hours.