KMgF3:Eu nanocubes with a mean edge length of ∼12 nm were synthesized by a hydrothermal method. Result of x-ray diffraction reveals that the nanocubes are perovskite phase. Under ultraviolet excitation, the broad emission bands from trace oxygen and color centers in KMgF3 matrix were observed. In comparison with a bulk sample having the same chemical compositions, no characteristic emissions of Eu2+ were observed, which can be attributed to the overlappment of the emissions of Eu2+ and color centers in KMgF3:Eu nanocubes. In addition, the emissions of Eu3+ were also detected, and the intensity ratio of 5D0 → 7F2 to 5D0 → 7F1 changed with excitation wavelength, indicating that the material has multiple luminescence centers or emitting states.