A modified criterion is developed on an empirical basis for the minimum thickness B min of a plane strain fracture toughness test specimen: B min = 400 K Ic 2 Eδ Y where K Ic is the plane strain fracture toughness, E is the Young's modulus and δ Y is the yield stress of the material. The modified criterion is tested alongside the ASTM thickness criterion against published data on the variation of K c with thickness, and shows significantly the better agreement with observed values of B min for a wide range of materials. An attempt has been made to rationalise this criterion. The expression is considered to take into account two major factors which determine B min , the attainment of plane strain in the specimen interior ahead of the crack tip, and the role of microstructure in determining how far the quasi-plane strain fracture (square fracture) extends beyond the region of true plane strain.