The microstructure of an AZ91D magnesium alloy mainly consists of Mg17Al12 phase and magnesium matrix. Although the Mg17Al12 phase is a principal precipitate in the Mg-Al alloy system, its role on corrosion behavior has not yet been systematically investigated. In the present study, the effect of Mg17Al12 phase on corrosion behavior of an AZ91D magnesium alloy was investigated in a quantitative manner. A wide range of volume fraction of the Mg17Al12 phase was obtained by varying aging temperature and time after solution heat treatment. The corrosion potential measurements and salt spray tests were carried out on the specimens with various volume fractions of the Mg17Al12 phase. The results of these tests showed that the corrosion resistance increased with increasing volume fraction of the Mg17Al12 phase. It was found that the network structure of the Mg17Al12 phase increased the corrosion resistance of an AZ91D alloy by acting as an effective barrier against corrosion.