Near-spherical Y2O2S:Yb3+,Ho3+ nanocrystals (NCs) with an average particle size of 40 nm were synthesized by the coprecipitation method followed by a solid-gas sulfuration technique. The effects of the Ho3+ ion doping concentration on the upconversion luminescence (UCL) property of the NCs was studied through the UCL spectra. Results show that the UCL intensity of Y2O2S:Yb3+,Ho3+ NCs markedly changes with Ho3+ ion concentration, and that the Ho3+ ion concentration quench is observed at 0.25 mol%. This value is only half as much as that in micron Y2O2S prepared by a solid state reaction, which can be attributed to the distinct diffusion mechanism of activator ions in the coprecipitation process. In addition, strong red emissions can be observed in Y2O2S:Yb3+,Ho3+ NCs throughout all Ho3+ doping concentrations used. However, the Ho3+ in micron Y2O2S usually exhibits weak red UCL. Infrared spectra confirm that this result is related to the large vibrational quanta produced by OH- and CO3 (2-) groups adsorbed onto the surface of NCs. These large vibrational quanta can remarkably increase the probability of S-5(2)-F-5(5) and I-5(6)-I-5(7) multiphonon relaxation, leading to the enhancement of red emissions arising from F-5(5) -> I-5(8) transitions. The UCL mechanism of the Yb3+-Ho3+ system in nano- and microsized Y2O2S is also discussed.