Ho3+–Yb3+ co-doped Y2O3 nanocrystals were synthesized by firing hydroxy carbonate precursors. Yb3+-concentration-dependent up-conversion properties of Ho3+ in Y2O3 nanocrystals have been investigated. The relative intensity of up-converted red emission increases more quickly than that of the green and the near-infrared ones with the enhancement of the concentration of Yb3+. It is believed that the energy process 5 S 2 (5F4) (Ho) + 5 I 7 (Ho) →5 I 6 (Ho)+5 F 5 (Ho) plays an important role in the population of the 5 F 5 level of Ho3+. The result indicates that the intensity ratio of the green emission to the red one can be tuned by changing the sensitizer concentration.