As-rolled microstructure and mechanical properties of a new kind of grain boundary allotriomorphic ferrite/granular bainite duplex steel plate and its crack propagation behavior were investigated in comparison with simple granular bainitic steel plate.These new duplex plate steels possess belter combination of strength and toughness than granular bainite plate steels under the conditions of conventional rolling and air-cooling.The observation of fatigue crack propagation behaviors showed that the existence of proper grain boundary allotriomorphic ferrite increases the compatible deformation ability of duplex microstructure and leads to the formation of crack branching and curving route,and it has an evidently blunting effect on microcrack tip and results in higher impact toughness.In addition,the new duplex steel plate also has good weldability as hot-rolled high strength low alloy structural steel. As-rolled microstructure and mechanical properties of a new kind of grain boundary allotriomorphic ferrite / granular bainite duplex steel plate and its crack propagation behavior were investigated in comparison with simple granular bainitic steel plate. These new duplex plate steels possession of belter combination of strength and toughness than granular bainite plate steels under the conditions of conventional rolling and air-cooling. The observation of fatigue crack propagation profiles showed the the existence of proper grain boundary allotriomorphic ferrite increases the compatible deformation ability of duplex microstructure and leads to the formation of crack branching and curving route, and it has an evidently blunting effect on microcrack tip and results in higher impact toughness. In addition, the new duplex steel plate also has good weldability as hot-rolled high strength low alloy structural steel.