Rod-like and flake-like up-converting Y2O3:Yb3+/Ho3+ particles which are composed of nanoparticles with size less than 100 nm, are prepared by a simple hydrothermal processing at 473 K (3 h) followed by additional thermal treatment at 1373 K (3 and 12 h). The effect of precursor pH value on the formation of Y2O3:Yb3+/Ho3+ is followed through X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Structural refinement confirms formation of the cubic bixbyte structure (S. G. Ia-3) with the non-uniform accommodation of dopants at C-2 and S-6 cationic sites. Under 978 nm laser excitation, strong green (530-570 nm) up-conversion is observed in all samples. The emission shows a decrease in intensity with an increase in external temperature, indicating FIR (fluorescence intensity ratio) based temperature sensing behavior of 0.52% for the F-5(4) -> I-5(8)/S-5(2) -> I-5(8) transitions.