An investigation has been performed to examine the fatigue crack propagation (FCP) rate and fatigue threshold of an Si-Mn steel containing carbide-free lathy bainite. Compact tension specimens prepared from this steel were given four different heat treatments to produce four different austenite contents. The fatigue test was carried out at stress ratio of -1 in a room temperature ambient atmosphere. The results show that the FCP threshold of the steel increases with an increase in the volume fraction of carbon-saturated austenite. The crack growth behaviours show that the deformation strengthening ability of the austenite has a significant effect on the FCP in the threshold region. The effect of austenite on the FCP threshold is seven times that of the bainite.