Development of luminescent materials doped with lanthanide ions, especially Er3+ may contribute to evolution of applications as ceramic laser active media, optical fiber amplifiers, fluorescence bioimaging agents, temperature sensing and up-conversion photovoltaic devices [1-3]. But the efficiency of such applications is influenced by dopant concentration which may affect the light emission properties. Understanding the energy transfer processes could clarify the optical mechanisms which take place in such materials pumped with light.  This work demonstrates the way in which the known green photoluminescence emission specific for 4S3/2→4I15/2, 2H11/2→4I15/2 transitions of Er3+ ions, is quenched due to the 2H9/2→4I13/2 cross relaxation at high concentrations of dopant (10 at%) in Y2O3 powders. The energy transfer mechanism of such a system, when pumped with λ=380 nm low-power light source, is proposed.