The effects of temperature and loading rate were investigated on the intiation of plane strain fracture of an HY-100 steel. Fracture toughness tests were conducted using pre-cracked notched round bars loaded in tension to produce two stress intensity rates, viz., K˙₁=1㎫√m/sec and K˙₁=2×10^6㎫√m/sec. Testing temperatures covered the range from -150℃ to 200℃ which encompassed fracture initiation modes involving quasi-cleavage to fully ductile fracture. Fracture processes were also examined by in-situ SEM observation under a wedge loading. The present steel contained the banded structures as well as MnS inclusions in the form of globulars and stringers aligned parallel to the rolling direction. The results show that this HY-100 steel was tougher under dynamic loading than under quasi-static loading, which was interpreted by correlating microstructures including MnS inclusions and the banded structures with fracture toughness and fracture processes.