Microstructure of an alloy has a significant effect on mechanical properties. Deep cryogenic treatment extends life of tool steels because of microstructure changes. In this research, effects of microstructural parameters were studied on tensile toughness of a medium carbon-low alloy tool steel. The results showed that the maximum population density of sub-micron sized secondary carbide was obtained after 36 h of soaking time. Also, amount of secondary carbides increased with soaking or tempering times from 2.18 vol% to 12.87 vol%. In addition, high population density and high content of secondary carbides were responsible for tensile toughness enhancement. Therefore, the best results (12-35% improvement in tensile toughness) were obtained for a specimen, which underwent a full treatment cycle consisting of heating, water quenching, soaking at $-196^{circ}C$ for 36 or 48 h and tempering at $200^{circ}C$ for 1 or 2 h, respectively.