The fatigue lifetime of the laser-MIG hybrid welded joint of 7075-T6 aluminum alloy was studied in this paper. The results showed that there existed a strong relationship between the fatigue crack initiation lifetime and fatigue failure lifetime. For the base metal specimens the ratio of fatigue crack initiation lifetime and fatigue failure lifetime was about 64.5%, while for the welded joint specimens it was about 20.2%. The observation of fatigue fracture surface showed that there existed a large number of dimples in the base metal specimens, while there appeared lots of gas porosities in the welded joint specimens instead. These gas porosities were regarded to be the reason of lower fatigue crack initiation lifetime.