NaYF4 doped with Ce3+ or Tb3+ or double-doped with Ce3+ and Tb3+ was prepared by the hydrothermal method. With the increasing reduction temperature, the hexagonal NaYF4 was gradually replaced by cubic NaYF4 (alpha-NaYF4). Tb3+-doped alpha-NaYF4 exhibited an intensive green photoluminescence under vacuum ultraviolet (VUV) or UV excitation. Although the energy transfer between Ce3+ and Tb3+ was not observed in the VUV region, the green emission of Tb3+ in alpha-NaYF4 was greatly improved with Ce3+ co-doped due to the energy-transfer procedure under UV excitation.