This paper shows the results of investigation of transformation behavior of steel 10Kh9V2MFBR. Continuous cooling transformation diagram was drawn up for steel 10Kh9V2MFBR by dilatometric analysis, hardness and electron-probe test and investigation of microstruсture. Experimental results show that at cooling rates more than 7,5 deg/min steel has a homogeneous structure of martensite with hardness about 434-494 HV. In case when the cooling rates in the range of 2,1...0,5 deg/min steel has a heterogeneous structure consisting of ferrite, pearlite, bainite and martensite, the hardness of steel falls to 182 HV. For semifinished products of steel 10Kh9V2MFBR the recommendation on selecting the quenching medium was provided, in case of section up to 200 mm the air was recommended as quenchant.