"We present our research on visible upconversion luminescence in Yb3+ doped Barium Zirconate (BaZrO3, BZO) under NIR excitation. Barium Zirconate was synthesized by hydrothermal method. The visible luminescence is attributed to both, Yb3+ cooperative emission and rare earth impurity upconversion emissions. Since the presence of Tm3+ and Er3+ impurities was not detected by absorption spectra, it has to be very low concentration. The strong intensity of all peaks is interesting considering the very small concentration. This fact suggests very efficient upconversion processes involving traces of Tm3+ and Er3+ ions having the Yb3+ ions as sensitizers. The emission band centered at 504 nm is twice the energy of the normal luminescence of a single ion, suggesting that it is the result of the radiative relaxation of an excited Yb3+-Yb3+ pair. For all Yb3+ concentrations, the lifetime for the Yb3+ cooperative emission is nearly half of the lifetime of single Yb3+ ion emission, suggesting the upconversion mechanisms are two photon uponversion processes. FTIR Spectra was recorded to indentify hydroxil groups, residual H2O and ionic bonds for the cubic crystalline structure of BZO. In addition, we present the thermodarkening effect in 0.5 at% Yb3+:BaZrO3 microcrystal (an increase of the UV-VIS edge band with annealing temperature). Both, the cooperative up-conversion emission under NIR excitation and fluorescence lifetimes of cooperative emission, increase with annealing temperature. Under UV and VIS excitation no emission of Yb2+ ions could be observed. This suggests that the broad band absorption in the UV-VIS region that increase with annealing temperature might be related to F color centers and oxygen vacancies which are responsible for the inhibition of the Yb2+ ion emission."