In this paper, the fabrication of fiber Bragg gratings by the fourth harmonic of a Nd:YAG laser is described. This is the first report of writing with this kind of ultravioletlight source. The spatial and temporal coherence of laser team is imptoved by applying a F-P etalon and a diaphragm of small aperture. The fiber was placed in an interferometer andexposed for 60 minutes. As a result, about 3% reflection was observed at 1505 μm.