A sample of YBa₂Cu₃O_7-x fluorinated by a high temperature (400 ℃) gas exchange technique has been investigated by means of ¹⁹F NMR (nuclear magneti resonance) measurements in both superconducting and normal state. As a result, behaviors characteristic of the superconductor, including a peculiarity in the spin-lattice relaxation rate (1/T1) around 250 K, were observed.