Hexagonal NaGdF 4 was synthesized by hydrothermal method at 240 o C. NaGdF 4 remains the hexagonal structure after heating treatment in argon atmosphere at 950 o C; however, transformation to the fluorite structure was observed after heating at 950 o C in air. The optical measurement in VUV range reveals different behavior for the hexagonal and fluorite forms. The excitation band of the Gd 3 + 4f-5d transition is located at about 120 nm for the hexagonal NaGdF 4 ; while for the cubic NaGdF 4 , it moves to lower energy (177 nm). It is concluded that the inserted oxygen atoms may play a role in the structure transformation. The Eu 3 + -doped hexagonal NaGdF 4 exhibits quantum cutting effect and emits two red photons (Eu 3 + ) upon excitation to the 6 G J level of Gd 3 + .