A supersmall punch test has been used to extract fracture strain information on irradiated 2.25Cr-1Mo steel from transmission electron microscopy disc specimens as small as 3 mm in diameter and 0.25 mm in thickness. The test is based on driving a steel ball punch through a clamped specimen. The size effect of biaxial equivalent fracture strain of various kinds of materials and irradiated steel has been demonstrated. The results of fracture strain obtained from specimens 3 mm in diameter has been related almost linearly to the fracture toughness J Ic for elastic and plastic behaviour. The relationship between fracture strain and fracture toughness J Ic has been verified for the irradiated nuclear pressure vessel steel 2.25Cr-1Mo so that large amounts of irradiation space in nuclear reactor could be saved.