Effects of specimen thickness and stress ratio on fatigue crack growth and crack closure levels under constant amplitude loading and after a single overload have been studied experimentally for a structural steel ( I 8G2A). The corresponding crack growth data from the fatigue tests have been presented and evaluated. The experimental trends have been compared to those reported in the literature for various steels. The ability of the effective stress intensity factor range based on crack closure measurements to correlate the observed crack growth response has been investigated.