Cavitation characteristics have been investigated at 653 K for Al-4.5Mg alloys containing of 0.09% and 0.2%Si. Large elongations above 350% were attained in both alloys. However. cavitation was increased by the large addition of Si because Mg2Si particles promoted cavity nucleation. Because cavity development reduced the mechanical property characteristics of the deformed specimens, it is important to investigate cavitation. Investigation for the sites of cavity nucleation revealed that the intragranular particles are more likely to nucleate cavities than the intergranular particles.