Abstract— The biaxial fatigue behaviour of two batches of a structural steel, termed isotropic and anisotropic batches, is examined. Tests were carried out using thin-walled hollow specimens subjected to axial load and internal and external pressure. The fatigue life of the isotropic batch is found to be about two to three times greater than the fatigue life of the anisotropic material. The ratio however is found to be dependent on the degree of biaxiality. Three equivalent strain parameters were used to correlate the fatigue lives, octahedral strain, Brown-Miller and Lohr-Ellison parameters. The difference between the theories are compared and the relation between the shear strain orientation and failure is discussed.