Lutetium oxide nanocrystals codoped with Tm3+ and Yb3+ have been successfully synthesized via adjusting the pH values of the precursor solution in a hydrothermal method followed by a subsequent calcination process. The samples were systematically characterized by X-ray diffraction, field-emission scanning microscopy, Fourier transform infrared transmittance spectroscopy, and upconversion luminescent spectra. The experimental results show that the pH values of the precursor solution have great effects on the structural, morphological, and upconversion luminescent properties of Lu2O3:2%Yb3+, 0.2%Tm3+ nanocrystals. The as-formed lutetium oxide precursors could transform to cubic Lu2O3 with the same morphology and a slight shrinkage in size after a calcination process. The upconversion emission intensity of Lu2O3:2%Yb3+, 0.2%Tm3+ nanocrystals obtained from the precursor solution with pH = 9 is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing the number of OH- groups and the enlarged nanocrystals size. Strong blue and weak red emissions from the prepared nanocrystals were observed under 980 nm laser excitation, which were attributed to the 1G4 --> 3H6 and 1G4 --> 3F4 transitions of Tm3+ ion, respectively.