In order to investigate the influence of oil on crack growth behavior, torsional fatigue tests were carried out on three kinds of age-hardened Al alloy specimens, 2017-T4 extruded, 20l7-T4 drawn and 7075-T6 extruded, with a small blind hole, in air and in oil. In both environments, the initial crack growth mode is the macroscopic tensile type in all materials. The crack leading to the final failure, however, is a shear-mode crack which is initiated after the initial tensile-type crack, except in the case of 7075-T6 extruded in air which propagates in the tensile mode. The crack growth rate is higher in oil than in air as a result of the lubricating action of the oil when the crack propagates in shear mode. On the other hand, this rate is decelerated in oil by the oil wedging action when the crack growth mode is the tensile type.