A series of Cd2+ doped hexagonal-phase NaYF4: Yb3+, Er3+ upconversion nanoparticles (UCNPs) were synthesized via solvothermal method, and the effect of Cd2+ on the upconversion luminescence (UCL) properties was investigated in detail. It was found that the introduction of Cd2+ could enhance the UCL intensities of NaYF4: Yb3+, Er3+ UCNPs significantly. The maximum enhancement occurred when the doping concentration of Cd2+ was 7 mol%. Compared with the UCNPs without Cd2+ doping, the total integral emission intensity was enhanced by 38 times. It was related to the unit cell expansion and the generated F-vacancies, which could decrease the crystal symmetry around the Er3+ ions. The results shall facilitate the improvement of UCL properties and the potential applications of UCNPs in lighting, solar cells, optics encoding and so on. (C) 2017 Elsevier Ltd. All rights reserved.