The effects of different filler sizes on the thermal properties were investigated in a thermal conductive composite (TCC) phase-change material (PCM) with three sizes (9, 65, and 300 nm) of silver nanoparticles (Ag NPs). Ag NP/paraffin composites (Ag/PW) were prepared by dispersing 0.5, 1.0, 1.5, and 2.0wt% of Ag NPs stably into molten paraffin using ultra-sonication and then solidifying the mixture. The thermal properties of the composite, such as the thermal conductivity, latent heat capacity, and thermal stability, were characterized by laser flash analysis (LFA), differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA), respectively. The degree and trend of the enhancement of the thermal properties differed based on the Ag NP size, and the efficiency increased with a decreasing particle size. In addition, we adopted some theoretical models to describe the behavior of thermal conductivity enhancement in this study. The results were explained by the difference in the interfacial area and degree of construct cluster of the Ag NPs, which were dependent on the particle size.