A set of hot rolled 7xxx aluminum alloys with different casting conditions and modification of processing conditions were analyzed on their fatigue life time as well as the internal microstructure using 3D computed X-ray tomography with a lab scale scanner and optical microscopy. It is shown that large retained casting pores exist in samples with low reduction rate. The modification on the hot rolling process is now proved to be much more effective on removing large casting pores in the alloy. It is also found that with the improved casting conditions and the additional deformation imposed by increased rolling reduction, this advantage is getting retarded. The combination of porosity analysis with low cycle fatigue test at peak stress of 300 MPa indicates that reduction of the size of large pores in the sample down to 10 µm will lead to greatly improved fatigue life times.