The gigacycle fatigue properties of two kinds of high strength low alloy steels were examined under axial loading, and the effect of micro-defect on their S-N properties and very slow crack growth behaviours was evaluated. Differing from the exhibited duplex S-N property for one of them under rotating bending, all data indicate the continuously descending S-N properties associated with surface and interior micro-defects induced failure under axial loading. The micro-defect causing interior failure corresponds to the microstructural inhomogeneity instead of the inclusion. Based on the evaluation of the severity of the stress distribution around the defect and crack tips, the surface or interior crack induced by the micro-defect can still propagate with a rate less than 10(-10) m/cycle in the gigacycle fatigue regime. Good agreement between the predicted and experimental results shows that the duplex S-N property and fatigue strength of steel in the gigacycle regime is greatly dependent on the size of the micro-defect exposed from the relevant control volume of specimen under different loading conditions, especially the size of the interior micro-defect.