Neutron irradiation embrittlement of 2¼Cr-1Mo steels for the pressure vessel material of the high temperature engineering test reactor (HTTR) is evaluated in terms of elastic-plastic fracture toughness, and the effects of neutron irradiation on thermal aging embrittlement is discussed. Plate materials including weld joints and forged materials were irradiated at temperatures of 290, 350, and 400°C up to a fluence of 4 % 1023 n/m2 (E > 1 MeV). Fracture toughness testing using disk-shaped compact tension specimens was conducted in the upper shelf region. Results show that fracture toughness degradation caused by 400°C irradiation has a different character than that imposed by irradiation at lower temperatures. The cause is explained in terms of irradiation-enhanced thermal aging embrittlement.