This study is carried out to investigate the ductile to brittle fracture behavior of special high strength low alloy steel, which is used in aerospace applications. As aerospace materials are subjected to temperature variations during their actual use therefore it is very important to know their fracture behavior with change of temperature. Alloy having 0.3%C, 0.88% Mn and 1.0% Si, Cr 0.89 % was heat treated to obtain tempered martensite and ferrite+tempered martensite structures. Impact toughness tests were carried out at different temperatures from – 60°C ~ 200°C in as received, fully quenched and tempered and quenched and tempered in two phase (a+ g) state. Then the effect of different Heat Treatments on Impact Transition Temperature was studied.