The extension of part-through cracks in thick-walled plates and tubes under fatigue loading has been investigated. Growth characteristics have been determined by analyzing the fracture surfaces. For this purpose artificial markings were induced on the fracture surfaces by an overload technique. The crack extension in the direction of the semiaxes as well as the crack growth along the whole front has been evaluated and interpreted in terms of the parameters: crack depth, ratio of crack width to crack depth, yield stress and the ratio of the stress intensity factor to the yield stress.