Abstract: The Al matrix composite foams reinforced by carbon nanotubes (CNTs) were fabricated by space-holder method. The effects of test temperature, frequency, amplitude, as well as porosity and CNT content of foams on damping properties were investigated by thermomechanical analyzer, meanwhile the corresponding mechanisms were analyzed systematically. The results show that damping properties increase with the increment of porosity and amplitude, but decrease with the increase of frequency. When the range of environmental test temperature is 25-200℃, the loss factor changes a little. As the temperature is higher than 200℃, the loss factor increases evidently with the increasing of temperature. CNT reinforcement can obviously improve the damping properties. At room temperature, the loss factor of 3.0%CNTs/Al composite foams is 0.27, which is 3.71 times as large as that of Al foams. The damping mechanisms are mainly dislocation damping, grain boundary damping, pore damping, CNTs' inherence damping and CNTs-Al interface damping. It should be noted that the damping of inherence and interface plays an important role in enhancing the damping properties.