Lutetium oxide nanocrystals codoped with Yb3+ and Tm3+ were synthesized by the hydrothermal method. Effect of the pH values of precursor solutions on the structure, morphology and upconversion luminescent properties of the nanocrystals was investigated. XRD results show that all the nanocrystals can be indexed to pure cubic phase of Lu2O3 with good crystallinity. Blue and red emissions from the prepared nanocrystals are observed at 980 nm laser excitation, which are located at 490 nm and 653 nm, respectively. The upconversion emission intensity of Lu2O3:2%Yb3+, 0.2%Tm3+ nanocrystals prepared at pH=9 is the strongest. The enhancement of the upconversion luminescence is suggested to be the consequence of the decreased number of OH- groups and the enlarged nanocrystals size. Power-dependent investigation reveals that the blue (490 nm) and red (653 nm) upconversion luminescence is a three-photon upconversion process