[en] KZnF₃:Er³⁺,Yb³⁺ was prepared by hydrothermal synthesis. For all samples (ErYb0-ErYb4), Er³⁺ concentration was fixed at 0.5 mol.%. Yb³⁺ concentration was changed from 0 to 4 mol.%. We observed upconversion luminescence at 653 and 553 nm (525 nm) when the samples were excited by 980 nm light. All samples were annealed at 400 deg. C under vacuum of 10^-5 Torr for 1 h. The upconversion intensity that was measured in the same condition changed when the sample was annealed. The upconversion mechanism of annealed samples is a two-photon process