The influence of the silicon nanocrystal (Si-nc) size on the erbium luminescence at 1.54 μm was studied in Er-doped silicon oxide thin films containing Si-nc. Er-doped and undoped SiO/SiO2 multilayers were prepared to control the Si-nc size. The results showed that the Er luminescence was due to an energy transfer process between Si-nc and Er ions. The proportion of Si-nc which couples with Er was dependent on the Si-nc size and the energy transfer process is maximal for a size equal to 4 nm. This preferentially coupling effect is interpreted by the existence of an optimum overlap between the Si-nc emission spectrum and the 4I9/2 multiplet of Er ions.