Rotating bending fatigue tests were carried out on specimens of an extruded aluminum alloy and a drawn aluminum alloy in air and in oil in order to investigate the effect of the oil environment on the fatigue strength. In the drawn aluminum alloy, the fatigue lives increase by the existence of oil under all stress levels. On the other hand, in the extruded aluminum alloy, the fatigue lives in oil and in air are nearly equal to each other under high nominal stresses, though the fatigue lives increase by the oil when the stress level is low. Moreover, S-N curves in the extruded aluminum alloy show the shape of the double stages in both environments. The crack propagates in the tensile mode, except for the extruded aluminum alloy under the high nominal stress, where the crack propagates in the shear mode, regardless of the environment. Therefore, the double stages of the S-N curve in the extruded aluminum alloy are caused by the change in the fracture mode depending on the stress level.