Molecular dynamics simulation was carried out to study the heat transfer behavior of the Al₂O₃/polydimethyl- siloxane (PDMS) composites, through analyzing the thermal conductivity, temperature gradient and the thermal conductivity enhancement, we discussed the effect of the radius and volume content on the thermal conductivity of Al₂O₃/PDMS composites. The results show that the thermal conductivity of the Al₂O₃/PDMS composites decreases first and then increases. Besides, when the particle radius is 5 nm, the thermal conductivity of Al₂O₃/PDMS composites attains the largest, which is higher than that with same volume content. In addition, by investigating the temperature and structure changes during the heat transfer behavior, we also found that the heat transfer of Al₂O₃/PDMS composites with smaller Al₂O₃fillers is mainly conducted in the form of phonons, the main influence factors that determine the thermal conductivity are heat capacity c, average velocity v and average free path l. Meanwhile, when the volume content is 15%, the conduction chains occur, the thermal can transfer successfully through the conduction chains formed by the Al₂O₃ composites, which is beneficial to the increases of the thermal conductivity. We also observed the change of the structure and surface meshes of the Al₂O₃/PDMS composites with the increasing of the volume content. The Al₂O₃ fillers increase, PDMS chains distribute separately, and the pore structure increases, promoting the rigidity of the molecules. Moreover, when the volume content is 15%, the PDMS chains are wrapped around the Al₂O₃ fillers, forming a PDMS-Al₂O₃ composite conductor, which enhances the synergistic effect of heat transfer, further explaining the conclusion that the Al₂O₃/PDMS composites with higher volume content has higher heat conduction. The whole system has similar change in the structure: i) with the increasing of the filler radius, the strip structure decreases, and the sphere structure increases. ii) the viewing of the partial enlarged drawing leads to the idea that the contribution of Al₂O₃ fillers to the conduction path increases with the increasing of the radius. iii) when the fillers are small, the distribution is more dispersed.