Abstract: In order to study the crack mechanism and fracture design theory of structural steel, generalized yield locus in the three-dimensional stress space is brought forward. Combining the theories of metal yielding and fracture in classical strength theory with those in damage mechanics, crack criterion of structural steel in the three-dimensional stress space is deduced based on the assumption of simultaneous yielding and fracture in the isotropic tensile stress field. In the fracture tests of rectangular steel plates with circular hole subjected to uniaxial tension with mean velocity, the load-displacement curves at initial cracking were accurately measured. Test results reveal that the first crack lies at the edge of the circular hole and spreads rapidly. By numerically simulating the stress field at initial cracking in the fracture tests, the applicability and accuracy of the crack criterion are comparatively verified.