An 1100 MPa grade ultra-high strength steel with different martensite fine structures,characterized by prior austenite grain size,martensite packet size,block width and lath width,was studied by various heat treatment processes.The result shows that with decreasing prior austenite grain size,both the packet size and block width decrease,while the lath width has virtually no change.Accordingly,both strength and toughness increase,while total elongation decreases.The yield strength has a Hall-Petch type relationship with the prior austenite grain size,packet size and block width,and the block width may be regarded as a key factor influencing strength.On the other hand,the ductile to brittle transition temperature(DBTT)is found to be more related to the packet size,which may be considered as a dominant factor influencing toughness. An 1100 MPa grade ultra-high strength steel with different martensite fine structures, characterized by prior austenite grain size, martensite packet size, block width and lath width, was studied by various heat treatment processes. , both the packet size and block width decrease, while the lath width has virtually no change. Accredially, both strength and toughness increase, while total decrease decreases. The yield strength has a Hall-Petch type relationship with the prior austenite grain size, packet size and block width, and the block width may be regarded as a key factor influencing strength. On the other hand, the ductile to brittle transition temperature (DBTT) is found to be more related to the packet size, which may be considered as a dominant factor influencing toughness.