The plane-strain fracture toughness test method enables the determination of the plane-strain fracture toughness (K IC ) of metallic materials by tests using a variety of precracked specimens, such as bend, compact tension (CT), arc-shaped, disk-shaped specimens. In this study, to determine the effect of the heat treatment process on the specimen thickness required for a valid plane-strain test in accordance with the ASTM standard, fracture tests are performed by using CT specimens made of AISI 1040 carbon steel. Furthermore, by taking into consideration the effect of residual stress caused by heat treatment in the vicinity of the notch, additional fracture tests are also performed. The specimens are machined from rolled plates in the L-T rolling direction (crack plane is perpendicular to the rolling direction). Having performed experimental analyses, the applicability of the plane-strain fracture toughness tests using CT specimen made of AISI 1040 steel is investigated.