In this paper, we investigate as a function of erbium concentration (1, 2, 5, 10 mol %) the upconversion properties of bulk and nanocrystalline Er3+ doped cubic Y2O3. After excitation at short wavelength (488 nm) or with upconversion pumping (815 nm), green and red emissions were observed for both nanocrystalline and bulk samples in the visible region of the spectrum, while blue emission was observed in bulk Y2O3:Er3+ only. The upconverted emission after 815 nm excitation revealed an enhancement of the red [F-4(9/2) --> I-4(5/2)] emission with respect to the green [(H-2(11/2), S-4(3/2)) --> I-4(15/2)] emission when the dopant concentration is increased. However, the magnitude of the red enhancement in the bulk material differs slightly from its nanocrystalline counterpart, with the nanocrystalline material showing a higher degree of dependence on the dopant Er3+ concentration. It is believed that two distinct mechanisms are responsible for populating the S-4(3/2) and F-4(9/2) levels, with the latter being more efficient at higher Er3+ ion concentration.