Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the prior austenite grain size ranging from 6μm to 199μm has been characterized by optical metallography(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a Hall-Petch relation with the yield strength.When the prior austenite grain size was refined from 199μm to 6μm,the yield strength increased by 235 MPa,while the Charpy U-notch impact energy at 77 K improved more than 8 times,indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength.The fracture surfaces implied that the unit crack path for cleavage fracture is identified as being the packet. Martensitic microstructure in quenched and tempered 17CrNiMo6 steel with the highest austenite grain size ranging from 6 μm to 199 μm has been characterized by optical metallography (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The yield strength and the toughness of the steel with various prior austenite grain sizes were tested and correlated with microstructure characteristics. Results show that both the prior austenite grain size and the martensitic packet size in the 17CrNiMo6 steel follow a Hall-Petch relation with the yield strength. When the prior austenite grain size was refined from 199 μm to 6 μm, the yield strength increased by 235 MPa, while the Charpy U-notch impact energy at 77 K improved more than 8 times, indicating that microstructure refinement is more effective in improving the resistance to cleavage fracture than in increasing the strength. the fracture surfaces implied that the unit crack path for cleavage fracture is identified as being t he packet.