The interlayer magnetic coupling of a Fe/Si/Fe trilayer shows an analogous feature to that of Fe/Si superlattices. With an increase in Si layer thickness, it oscillates as ferromagnetic (first F), antiferromagnetic (AF), ferromagnetic (second F), and finally reaches a noncoupling (N) state. We have investigated interlayer coupling of Fe/Si/Fe trilayers inserting very thin (1 or 2 ML thick) boundary layers X (X=Ag, Ge, Fe–Si, Ta, etc.). They are expected to suppress interatomic diffusion between Fe and Si layers. Interlayer coupling of Fe/X/Si/X/Fe with negligible interdiffusion is simply F and changes to N as the Si layer thickness increases. Furthermore, Fe/Fe–Si/Fe trilayers which show coupling of first F, AF but not second F, reproduce second F when a Si layer is inserted in the Fe–Si spacer. These results imply that an amorphous Si spacer mediates ferromagnetic coupling between neighboring Fe layers while the first F and the strong AF coupling usually observed in Fe/Si superlattices are caused by diffused crystalline Fe–Si.