This study is concerned with the fracture characteristics of RQT 701; a possible contender for the replacement of HY 100, the existing 690MNm-2 (100,000psi) strength level Naval quality steel. RQT 701 is a low nickel, boron containing quenched and tempered steel which is less expensive than HY 100 due to a lower nickel content. The hardenability is maintained by the boron addition. The preliminary testing of RQT 701 showed a variation in the impact transition temperature through the thickness of the plate. This study examines the possible causes of this variation in impact properties and makes a full assessment of the mechanical properties of RQT 701. A full metallographic investigation has been carried out using quantitative metallography, scanning electron microscopy and transmission electron microscopy. The effect of heat treatment on the steel's hardenability and mechanical properties has been studied so that suggestions to produce optimum properties of RQT 701 could be made. A full fractographic study has also been undertaken. The relationship between fracture characteristics, microstructure and hardenability has been discussed. A parallel investigation of HY 100 plate has been carried out. A comparison between the microstructures and fracture properties has been made and the relative merits of the two steels discussed.