beta-NaGdF4:Yb3+, Er3+ upconversion (UC) microcrystals were prepared by a facile hydrothermal process with the assistance of ethylene diamine tertraacetic acid (EDTA). The beta-NaGdF4 UC microcrystal morphology was controlled by changing the doses of EDTA and NaF. Uniform hexagonal structure can be obtained at the 2 mmol EDTA and 9-10 mmol NaF. The UC emissions of beta-NaGdF4:Yb3+, Er3+ microcrystals were tuned by the variation of Eu3+ doping level (0%-5%), where the red/green intensity ratio decreased with the Eu3+ concentration increase. It was found on the base of rate equations that with the Eu3+ doping, the energy back transfer process H-2(11/2)/S-4(3/2) (Er3+) -> I-4(13/2) (Er3+) decreased. In addition, an energy-transfer process from F-4(7/2) (Er3+) to D-5(1) (Eu3+) and a cross relaxation process of H-7(9/2) (Er3+) + D-5(0) (Eu3+) -> F-4(7/2) (Er3+) + D-5(2) (Eu3+) were proposed and verified by rate equations, which dominated the energy-transfer mechanism between Er3+ and Eu3+, resulted in the spectra tuning of beta-NaGdF4:Yb3+, Er3+. The results suggested that the color tuning of beta-NaGdF4:Yb3+, Er3+, Eu3+ UC microcrystals would have potential applications in such fields as flat-panel displays, solid-state lasers, and photovoltaics.