The Jlc fracture toughness behavior of unirradiated and irradiated Mo steel plate and weldment was characterized by the multiple-specimen R curve technique using 0.577 in.-thick (T), 1-T, and 2-T compact specimens. At room temperature, the unirradiated plate exhibited limited plastic deformation and then failed catastrophically due to unstable crack growth in the transition temperature regime. At 427°C, the Mo base metal failed in a stable crack growth mode, and the Jlc value was twice that obtained at room temperature (145 versus 70 to 90 kJ/m2). The weldment was found to be very resistant to unstable tearing at both 24 and 427°C. Its Jlc response, 175 kJ/m2 at 24°C and 116 kJ/m2 at 427°C, was superior to that of the plate at room temperature, but slightly lower than the base metal toughness at 427°C. The effect of specimen size on the elastic-plastic fracture toughness response of the plate and weldment was characterized at 427°C. The 0.577-T and 2-T plate specimens were found to yield comparable Jlc values; however, the smaller specimen exhibited a steeper R curve and higher tearing modulus. The 0.577-T and 1-T weld specimens yielded comparable fracture toughness properties. The JIc fracture toughness for both the plate and weldment was reduced by ∼20% as a result of irradiation to total fluences of 3.2 × 1021 to 5.0 × 1021 n/cm2. The tearing resistance of the plate was found to be insensitive to irradiation, but a fourfold degradation in the tearing modulus was observed in the irradiated weldment.