Abstract: A new hot-rolled low alloy high strength steel with grain boundary allotriomorphic ferrite/granular bainite duplex micro-structure has been developed through novel microstructure and alloying designs without any noble metal elements such as nickel and molybdenum. Its as-rolled microstructure and mechanical properties, fatigue crack propagation behavior compared with single granular bainitic steel as well as continuous cooling transformation, were investigated in detail. The measured result of CCT (continuous cooling transformation) curve shows that such duplex microstructure can be easily obtained within a wide air-cooling rate range. More importantly, this duplex microstructure has much better combination of toughness and strength than the single granular bainite microstructure. It is found that the grain boundary allotriomorphic ferrite in this duplex microstructure can blunt the mi-crocrack tip, cause fatigue crack propagation route branching and curving, and thus it increases the resistance to fatigue crack propa-gation, improves steel toughness. The mechanical properties of the above commercial duplex steel plates have achieved or exceeded 870 MPa ultimate tensile strength, 570 MPa yield strength, 18% elongation and 34 J Charpy V-notch impact energy at -40℃,showing good development potential.