Abstract Monodispersed spherical Y 2 O 3 :Yb, Ho upconversion luminescence (UCL) particles with sizes of 40–200 nm are prepared using a homogeneous precipitation method. It is found that aging time, varying between 90 and 10 min, has a profound influence on the precursor size, which systematically decreases from 230 nm to 50 nm. The precursor shows poor stability when aging time is 10 min, and the stability of precursor can be improved by increasing the urea concentration. The UCL spectra of Y 2 O 3 :Yb, Ho with different particle sizes are investigated. The results indicate that the integrated emission intensity ratio of green to red ( R green/red ) exhibits a gradual decrease from 2.7 to 0.45 when the particle size decreases from 200 nm to 40 nm, and the possible reasons are evaluated.