ZnO:Er 3+ and ZnO:Er 3+ –Yb 3+ nanoparticles (NPs) are fabricated by a sol–gel method, afterwards parts of which are separated and surface modified in Mo(NO 3 ) 3 solution. Analyses on phase and structure based on X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques indicate that Er 3+ and Yb 3+ are incorporated into the ZnO lattice successfully and after Mo treatment, a thin layer of MoO 3 forms on the NPs surface, forming core/shell structures. Raman scattering spectra reveal the existence of ZnMoO 4 in the shell part. Visible up-conversion (UC) is observed in all the samples, with Mo treated and untreated ZnO:Er 3+ emitting dominant but relatively weak red light, corresponding to 4 F 9/2 – 4 I 15/2 transition of Er 3+ . In Yb 3+ -codoping systems, the integral UC intensity is enhanced obviously though red emission still dominates the UC spectra before surface modification. In the Mo treated system, ZnO:Er 3+ –Yb 3+ /MoO 3 , green emission is increased while the red is suppressed in comparison to ZnO:Er 3+ –Yb 3+ , with the intensity of green to red ratio (GRR) changing from 0.25 to 8. A novel phenomenon is discovered that the green emissions in our samples involve three-photon processes.