Abstract-Strong visible green upconversion emission in nanocrystalline BaZrO 3 :Yb 3+ powder, obtained by a hydrothermal process at 100 • C, is reported. The unconverted emission has a quadratic dependence on the pump intensity with a lifetime around half that of the NIR lifetime. Results suggest cooperative upconversion as the mechanism responsible for the green fluorescence. This efficient Yb 3+ -based cooperative up-conversion process allows the development of novel emitting materials in the ultraviolet to visible (UV-VIS) range.