A covering flux is commonly used to prevent an aluminum melt from reacting with the surrounding atmosphere or from re-oxidizing. In this study, melts were degassed with and without a covering flux using a porous bar diffuser. After degassing and holding, the melts were then poured to obtain chilled samples, reduced pressure samples and permanent mold castings. The chilled samples were polished and treated by ultrasonic vibration to reveal any foggy marks and the area of the foggy marks and the pore count were measured. The densities of the chilled samples and the reduced pressure sample were also measured to compute the relative porosities. The factors influencing the relative porosities of the aluminum alloy castings were then discussed. Rotational bending fatigue tests were also conducted to assess the effect of the pore count and the relative porosity on the fatigue life cycles of the A356 alloy castings.