ZnO nanocrystals doped with trivalent europium ions (Eu³⁺) and dysprosium ions (Dy³⁺) were synthesized by the precipitation method. The structural and optical properties of the samples are investigated by the X-ray diffraction (XRD) and photoluminescence (PL). The results show that rare earth ions are incorporated into the lattice of ZnO, and the combination of blue, green and red emissions can be obtained. Specially, the emission can be obtained even under the nonresonant excitation of 320 nm, which is explained based on the energy transfer. The concentration quenching mechanism is also presented in this paper.