In this study, the effect of thickness on the fracture toughness of the steel plate with the thickness of 80mm has been investigated by the wide plate tensile test and ESSO test. The fracture toughness for crack initiation and propagation was evaluated quantitatively for the full thickness specimen. It was found that EH-36 grade steel with the thickness of 80mmt showed the KIC value of 164kgf/mm1.5 at -145°C. Also, large-scale ESSO test result showed that the steel with the thickness of 80mm had 520kgf/mm1.5 at -10°C. Although it was known that the fracture toughness decreases with the increase of the plate thickness, EH-36 grade steel with the thickness of 80mm had enough values of fracture toughness to prevent the crack initiation and arrest the brittle crack propagation.