The high strength low alloy (HSLA) ۱۰۰ steel has been widely used in various industries such as shipbuilding, military, oil and gas. An increase in yield strength with a reasonable toughness is highly demanding in this steel. In the present study, the quench-tempering heat treatment was investigated to improve mechanical properties of this steel. The samples were austenitized at ۹۳۰ °C for ۱ h followed by water quenching. They were then tempered in the temperature range of ۴۱۰ and ۷۱۰ °C for ۱ to ۴ h. Microstructures of the heat treated specimens were evaluated by optical and scanning electron microscopy, while mechanical properties measured by hardness, tensile, and toughness tests. The results showed that microstructural changes during tempering took place mainly after ۶۵۰ °C, where the martensite laths and acicular ferrite transformed to polygonal ferrite. The yield and ultimate strengths were increased by increasing tempering temperature from ۴۱۰ to ۵۲۵ °C, beyond which both was dropped up to ۷۱۰ °C. The maximum yield and ultimate strengths were obtained by tempering at either ۵۲۵ °C for ۱ h or ۴۷۵ °C for ۲ h. Increasing tempering temperature and duration both resulted in an increase in the absorbed impact energy.