Specimens of different thickness made of a low-alloy steel were tested to determine fracture toughness under static and impact loading at room and low temperatures. It was found that for the specimen of each thickness there was a specific brittle-to-ductile transition temperature Tps defined as the upper temperature boundary up to which plane-strain conditions were valid at fracture. The temperature Tee, as well as the brittleness transition temperature Ts0 (determined via the 50% ductile component of the fracture surface), rose with the increase in the thickness B of a specimen. The value of Tps was found to be in a linear dependence on lgB for both static and impact loading.