On-line thermo mechanical controlled processing(TMCP) was conducted to develop the third generation high strength low alloy(HSLA) steel with high toughness economically.The ultra-low carbon content ensured a high level of upper shelf energy while ultrafine lath martensitic structure transformed from super-thin pancaked austenite during controlled rolling and cooling.The reduction of martensite block size decreased ductile-to-brittle transition temperature(DBTT) and compensated the strength loss due to carbon reduction.Consequently,the excellent balance of strength and toughness values was obtained as 950-1060 MPa for yield strength,180 J for Charpy V-notch impact absorbed energy at 30℃,which is much superior to that of traditional martensitic steel.Two mechanisms for the refinement of lath martensite block were proposed:One is the austenite grain refinement in the direction of thickness,and the other is the reduction in the fraction of sub-block boundaries with small misorientation and the increase in the fraction of block boundaries with large misorientation,possibly due to austenite hardening. On-line thermo mechanical controlled processing (TMCP) was conducted to develop the third generation high strength low alloy (HSLA) steel with high toughness economically. Ultra-low carbon content ensures a high level of upper shelf energy while ultrafine lath martensitic transformed from super-thin pancaked austenite during controlled rolling and cooling. the reduction of martensite block size decreased ductile-to-brittle transition temperature (DBTT) and compensated the strength loss due to carbon reduction. Conclusion, the excellent balance of strength and toughness values ​​was obtained as 950-1060 MPa for yield strength, 180 J for Charpy V-notch impact absorbed energy at 30 ° C, which is much superior to that of traditional martensitic steel. Two mechanisms for the refinement of lath martensite block were proposed: One is the austenite grain refinement in the direction of thickness, and the other is the reduction in the fraction of sub-block boundaries with small misorientation a nd the increase in the fraction of block boundaries with large misorientation, possibly due to austenite hardening.