In order to solve the problem that the brittle cracking accidents occasionally occur for the thick plates in steel structures,the relationship between the thickness and fracture toughness of steel was investigated.With adopting the three-point bending(TPB) specimens,the crack tip opening displacement(CTOD) tests for thick Q345B steel plates used for structures were carried out at low temperature.The different steel plate thickness,test temperatures and sampling locations along the thickness direction were s...