Abstract: This article deals with the effect of heat treating processes on microarea chemical composition, Micro-structure and fracture properties of ultrahigh strength steel 37 SiMnCrNiMoV. The condition of formation of twined martensite in the steel and effect of its distribution on fracture properties were examined by TEM observation, electron microprobe analysis and imitative calculations. It was emphasized that microarea chemical composition seems to be the most important microsstrctural parameter in determining the morphology and distribution of martensite on which depends the strength and toughness of the steel. The contributions of various microstructural details to strength and toughness of the steel was analysed and a new"inherit ing"heat treatment was developed with which the KEC value of the steel can be increased by 25%, in comparison with commercial heat treatment, and the Ak value of the steel can be increased by 65%, in comparison with ultrahigh temperature quenching.