This study was carried out to determine the minimum Al content needed to form an Al2O3 scale on creep resistant ferritic steels at 650 °C. Two steels differing mainly in Al content were oxidized in air at 650 °C for 3000 h. One of the steels contained 2.3 wt% Al and the other 1.9 wt% Al. Oxidation resistance of the two steels was also compared with that of the commercial P92 steel at the same temperature. The oxidation was monitored by weight gain measurement. XRD, SEM and EDS techniques were used to analyze the scale formed on the surface of the steels. For the steel containing 2.3 wt% Al, a continuous Al2O3 scale was observed after 3000 h of oxidation and growth of the scale was parabolic with an extremely low rate constant of 0.00058 mg cm-2 h-1/2. For the steel containing 1.9 wt% Al, however, only a non-protective scale was formed, which exhibited a layer structure that consisted of an outermost porous Fe2O3 layer, followed by a relatively dense intermixed Fe2O3 and FeCr2O4 inner layer and then by an internal oxidation layer containing voids, Al2O3 and un-reacted metal particles in addition to Fe and Cr oxides; growth of this type of non-protective scale followed the logarithmic kinetics Δmt = klln(αt + 1) for oxidation times up to 3000 h.