The effect of filler surface treatment on the dynamic rheological behaviors of polystyrene filled with Sn-Pb alloy particles was tested below and above the melting temperature (T-m) of the alloy. The mechanical relaxation relevant to the T-m of the alloy in the composite was diminished by the filler surface pretreatment. In the whole temperature range of interest, there existed a secondary plateau of the storage modulus at low frequencies. The effect of alloy particle surface treatment on the plateau was related to the matter-state change (from solid to liquid) of the alloy. Above the T-m of the alloy, the surface treatment of the alloy affected the secondary plateau, but below the T-m, it did not. The analyses of Cole-Cole diagrams of the systems suggested that untreated and pretreated alloy fillers all retarded the relaxation processes in the molten state of polystyrene below the T-m of the alloy and that the relaxation process was separated into the high-frequency relaxation of the phases and the low-frequency relaxation of the droplets above the T-m. The surface treatments of the alloy filler further enhanced this action.