Fatigue phenomena in conventionally sintered Al-5%Cu-0.5%Mg-0.8%Si alloy with a density ratio of 91.5% has been investigated by using a cantilever type internal friction measurement device which is electro-magnetically drived and vibrated at the resonance frequency(f_r). Changes in f_r during vibration at a constant amplitude correspond to those in the internal friction(Q^-1), and a marked decrease in f_r occurs as a result of the formation of fatigue cracks. These cracks propagate mainly along grain boundaries in as-sintered specimens, and transgranularly in T6 treated ones.