The fatigue properties of high-pressure die-casting (HPDC) magnesium (Mg) alloys AZ91 exhibit a high variability, due primarily to the porosity that is inherent in the injection process. In the 94% of the studied samples, the porosity in which crack nucleation originates is at the surface or adjacent to the surface. The threshold stress intensity factor amplitude and the limit of fatigue have been calculated following the classical models of parameterization of defects. A new set of samples were prepared by machining the surface slightly, in order to conserve the microstructure, and the fatigue behavior at low level of stress was improved. All the samples were produced in molds with the final shape by HPDC process, which allowed a realistic study of the surface effect and the influence of grain size variation from the edge to the center of the samples.