Yb3+/Er3+ co-doped Gd6WO12 nanoparticles were prepared via a coprecipitation technique and characterized by means of XRD and FE-SEM. The effect of excitation power density in different regions on the upconversion mechanisms was investigated. It was found that the green upconversion emissions is a two photons process when the excitation power density is low, but a three photons process occurs when the excitation power density is high. However, the red upconversion emission is still a two photons process. A possible mechanism for three photons green upconversion luminescence was proposed and discussed. (C) 2012 Elsevier B.V. All rights reserved.