Two-dimensional (2D) ferromagnetic materials have been exhibiting promising potential in applications,such as spintronics devices.To grow epitaxial magnetic films on silicon substrate,in the single-layer limit,is practically important but challenging.In this study,we realized the epitaxial growth of MnSn monolayer on Si(111) substrate,with an atomically thin Sn/Si(111)-2√3 × 2√3-buffer layer,and controlled the MnSn thickness with atomic-layer precision.We discovered the ferromagnetism in MnSn monolayer with the Curie temperature (Tc) of ～54 K.As the MnSn film is grown to 4 monolayers,Tc increases accordingly to ～235 K.The lattice of the epitaxial MnSn monolayer as well as the Sn/Si(111)-2√3 × 2√3 is perfectly compatible with silicon,and thus an sharp interface is formed between MnSn,Sn and Si.This system provides a new platform for exploring the 2D ferromagnetism,integrating magnetic monolayers into silicon-based technology,and engineering the spintronics heterostructures.