The La（OH）,2, Br·1, 5H,2,O and La（OH）,2,Br·H,2,O were obtained under ordinary temperature and pressure by the lanthanum oxybromide reacting with water or water vapor respectively, and the La（OH）,2, Br was obtained by heating La（OH）,2, Br·1.5H,2,O or La（OH）,2,Br·H,2,O at 120—140℃. The thermal decomposition processes and crystal structures of these compounds were investigated by using D.T.A., T.G.A. and powder X-ray diffraction methods. The La（OH）,2, Br·1.5H,2,O loses one crystal water at 108—140℃ and half a crystal water at 140—210℃, but the La（OH）,2, Br·H,2,O loses crystal water at 125—226℃, to form La（OH）,2,Br. Then the La（OH）,2, Br decomposes thermally at 310—390℃ under N,2, stream to form LaOBr. The La（OH）,2,Br·1.5H,2,O forms hexagonal crystals with a=8.4350A, c=4.2302A, and Z=2. The La （OH）,2, Br·H,2,O forms orthorhomhic crystals with a=12.635A, b=5.023A, c=17.275A, and Z=10. The La（OH）,2, Br forms monoclinic crystals with a=6.3742A, b=4.0306A, c=7.1599A,β=113.121°, and Z=2. The IR spectra of these compounds were determined and discussed.