The influence of nonmetallic inclusions on the fracture properties of A588 Grade A steel plate is investigated by evaluating one plate produced by conventional steel-making practices and another using calcium treatment. The two plates had differing inclusion structures and their characterization included: tensile, Charpy V-notch, dynamic tear, and J-integral testing; J_Ic, tearing modulus, and J-Δa and J-T curve determinations on the upper shelf; and fatigue crack propagation and near-threshold fatigue evaluations. Testing of up to six specimen orientations was used, depending on the testing method. Of all the test methods used in this investigation the J_Ic and J-T curve determinations were found to be the most sensitive to changes in inclusion structure in all testing orientations. The fatigue crack growth rate of the calcium-treated steel was primarily improved in the through-thickness (SL) orientation at higher ΔK levels. Testing at higher load ratios accentuated these differences. Oxide and roughness-induced crack closure had a significant effect on the near-threshold fatigue behavior of the conventional steel as a result of the inclusion-induced roughness on the fracture surface. Fractography and crack closure measurements were used to explain this behavior.