Effect of humidity on the growth mechanism of a fatigue crack in an age-hardened Al alloy was investigated in relative humidity environments of 25% and 85% at loading frequencies of 50Hz and 6Hz. Macroscopic growth mode of a crack in low humidity was a tensile one irrespective of loading frequency. By contrast, the mode in high humidity was changed from the shear one to the tensile one with increase in fatigue life at 50 Hz, while it was the tensile mode only in the wide range of fatigue life at 6 Hz. In low humidity, the crack propagated by forming striations which are general in fatigue of many metals. On the other hand, in high humidity, most of fracture surface was occupied by many slip planes in the shear mode crack and by granular facets in addition to striations in the tensile mode one. The crack growth rate in high humidity was higher in the shear mode crack than in the tensile mode one. The differences in growth mechanism and growth rate of a crack were explained by the content of diffused hydrogen generated by reaction of water vapor and aluminum in addition to the microstructure with sub-grain and texture and the cyclic softening behavior of the alloy.