Sub-micronic, spherical Y2O3:Yb/Er particles comprising clustered nano-units (70 nm) were prepared via ultrasonic spray pyrolysis from pure nitrate precursor solutions with different Yb/Er dopant ratios. The particles were additionally thermally treated at 1100 °C for 12, 24 and 48 h. The structural and morphological characteristics of particles were studied by X-ray powder diffraction, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, energy dispersive X-ray analysis and specific surface area (BET) and were further correlated with their advanced optical properties. For the recorded up-conversion emissions, originating from the following Er3+ transitions: [2H9/2→4I15/2] in blue (407-420 nm); [2H11/2, 4S3/2→4I15/2] green: 510-590 nm; and [4F9/2→4I15/2] in red (640-720 nm) spectral region, the corresponding lifetimes were acquired in the wide temperature range (10-300 K). The most intense green up-conversion emission with the long decay of 550 ms is recorded for Y1.97Yb0.02Er0.01O3 particles thermally treated at 1100 °C for 24 h.