Abstract In this study, monoclinic luminescent Gd 2O 3 nanocrystals doped with different concentrations of Er 3+ (0.1, 1, and 10 mol%) were produced by propellant synthesis and flame spray pyrolysis (FSP). A comparison of their optical and morphological properties is reported. Following 980 nm excitation, an increase of the emission intensity from the 2H 11/2, 4S 3/2 → 4I 15/2 and 4F 9/2 → 4I 15/2 transitions was observed with increasing Er 3+ concentration in the Gd 2O 3 nanocrystalline samples prepared via both techniques. However, the overall upconversion emission intensity was greater for the samples obtained by FSP. Furthermore, as the Er 3+ concentration was increased, the intensity of the red ( 4F 9/2 → 4I 15/2) emission was observed to increase more rapidly in comparison to the green ( 2H 11/2, 4S 3/2 → 4I 15/2) emission resulting in an overall enhancement of the red component in the upconversion emission. Although both synthetic routes yield average crystallite sizes in the nanoscale, the TEM and SEM images confirm a more homogeneous morphology and lower particle aggregation for the nanocrystals produced by FSP.