Mg2+ doped KMnF3:Yb3+/Er3+ nanoparticles are synthesized by solvothermal method. The single-red-band upconversion emission is observed in all samples. A maximum 20.8-fold UC luminescence enhancement is obtained due to the introduction of Mg2+. Monodisperse Mg2+ doped KMnF3:Yb3+/Er3+ NPs show a biocompatible ultra-small size with 10 nm.