<p>There are conflicting results and doubts whether case-hardening treatment could still be effective in improving the fatigue performance of high-strength steels in the very-high-cycle fatigue (VHCF) regime. This study aims to explore the VHCF behavior of a case-carburized low-carbon bearing steel by ultrasonic fatigue testing under axial loading. The results show that all the failures of the carburized sample were initiated from interior defects of inclusions and steel matrix with their initiation sites beyond the case layer in the life regime beyond ~ 10⁶ cycles, which is quite different from that of the conventional quenched and tempered (QT) sample which exhibited both surface and interior failure modes. As a result, the fatigue strength at 10⁹ cycles of the carburized sample (577 MPa) is lower than that of the QT sample (654 MPa) by ~ 11.8%, indicating that the carburizing treatment loses its expected effect on enhancing the fatigue strength of the experimental steel in the VHCF regime. This difference was primarily because of the change of the actual true stress distribution across specimen section due to the generation of compressive residual stress and the grain coarsening after the carburizing treatment.</p> Graphical abstract <p></p>