Internal friction measurements have been performed at the different frequencies, temperatures and strain amplitudes to examine the damping characteristics of foamed aluminum. It is found that the pores contribute to the damping by raising the internal friction background, particularly the high temperature one. The other significant feature is that the internal friction is strain-amplitude-dependent, which becomes more obvious as the pore size decreases. There are inhomogeneous pores and high density of defects in the structure, leading to nonuniformly distributed stress and strain. This state will be intensified under the disturbance of applied load, making the defect density increase. The defect effects are therefore responsible for the damping characteristics of foamed aluminum.