In this paper, we observed that in Er 3+ /Yb 3+ codoped nanocrystals (NC), with the decreasing particle size and the increasing Yb 3+ concentration, the upconversion luminescence (UCL) of the red emissions of F9/24−I15/24 gradually increased, while the green emissions of S3/24/H11/22 gradually diminished under 980nm diode laser excitation. In NC with lower Yb 3+ concentration, both the red and green emissions result from a two-photon excitation. In NC with higher Yb 3+ concentration, the red emissions result from a two-photon excitation, while the green emissions dominantly result from a three-photon excitation. A model was provided for explaining the above UCL phenomena.