The effect of surface treatment on the corrosion behavior of diecast magnesium alloy AZ 91D-F has been investigated by salt spraying test. It was found that the corrosion resistance of the alloy decreased in the following order of surface treatments: anodizing with sealing, chromating, anodizing without sealing, mechanical polishing and no treatment (as-cast). In any case, both the weight loss and the estimated maximum corrosion depth increased linearly with increase in the duration of spray test. X-ray diffraction analysis revealed that corrosion products consisted mainly of Mg(OH)2 and NaCl, irrespective of surface treatments. Polarization characteristics in 5%NaCl aqueous solution, however, depend on the surface states and are correlated well with the order of corrosion resistance.