Effect　of　tempering　temperature　on　the　microstructure　and　mechanical　properties　of　isothermal　quenched　new　building　steel　sheets　was　studied　by　means　of　metallographic　microscope,　scanning　electron　microscopy,　transmission　electron　microscopy　and　tensile　testing　machine.　The　results　show　that　the　as-quenched　specimen　consists　of　acicular　lower　bainite　and　retained　austenite.　After　tempering　at　235-435　℃,　the　retained　austenite　content　in　the　matrix　decreases　with　the　tempering　temperature　increasing,　and　the　slab　interface　of　lower　bainite　is　gradually　blurred　with　the　tempering　temperature　increasing.　After　tempering　at　235-435　℃　for　1　h　both　the　tensile　strength　and　yield　strength　of　the　tested　steel　decrease　in　varying　degrees,　but　the　elongation　increases　significantly.　The　strength-plastic　product　increases　first　and　then　decreases　with　the　tempering　temperature　rising,　the　maximum　value　is　obtained　at　285　℃　(34　017　MPa•%).　The　impact　absorbed　energy　by　tempered　steel　at　room　temperature　is　higher　than　that　of　isothermal　quenched　steel,　and　the　impact　toughness　of　tempered　steel　is　better　when　tempering　temperature　is　lower　than　385　℃,and　the　impact　fracture　morphology　is　consistent　with　the　impact　test　results.　©　2019,　Editorial　Office　of　＂Jinshu　Rechuli＂.　All　right　reserved.