Vibration damping capacity(i.e Specific Damping Capacity, SDC) of extruded bars of both casted and Powder Metallurgy(PM) Al-Zn alloys was investigated in relation to their Zn compositions and heat treatment conditions. The effect of α/β interface area on SDC was studied. The SDC is found to increase with increasing Zn content. The SDC of specimens heat treated at 250℃ for 1 hour is smaller than that of as-extruded bars because their α/β interface area decreases. In the case of extruded bars of PM Al-78wt%Zn alloy heat treated at 250℃ and 310℃ for 1 hour, SDC and lamellar spacing are 12.8% and 0.5㎛, 25.6% and 0.1㎛, respectively. This large difference in SDC is believed due to the disappearance of retained α phase as well as much finer lamellar spacing by heat treating at 310℃. The SDC of extruded bars of PM alloys is about 3% larger than that of extruded bars of casting.