Two kinds of poly(methyl methacrylate)/gallium (PMMA/Ga) nanocomposites with different Ga contents (11.3% and 13.5%) were prepared by free radical polymerization. The relaxation dynamics of PMMA/Ga nanocomposite above the glass transition temperature has been investigated by mechanical spectroscopy. It was found that the peak temperatures of α relaxation of the nanocomposites increase with the increasing Ga content, but the peak heights of α relaxation decrease. Besides, the composition-dependent dynamics of the α' relaxation in PMMA/Ga nanocomposites was also studied.