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A family of ultra-low carbon bainitic (ULCB) steels for heavy plateapplication has been investigated. These new materials have been developed as candidatesto replace traditional heat treated high yield strength (HY) steels. The ULCB steels achievehigh fracture toughness through the use of advanced alloy design and thermomechanicalprocessing. This paper describes how the metallurgical factors such as the chemicalcomposition, austenite conditioning and steel cleanliness influence on fracture toughness ofthe material using J-integral method. In addition, utilizing scanning electron microscopymethods, the basic modes of fracture in the ULCB steels have been examined.