Isotopic composition of oxygen in Gejiu granites is characterized by abun dance of 18O, which indicated that the source material was derived from the shallow sedimentary crust.In consideration of the fact that the isotopic composi- tions of oxygen and hydrogen in magma liquid are similar with that of the "magmatic water" defined by Taylor the tin-bearing granites should be classi- fied genesically as an anatexis magmatic-granite in the shallow source (it cor- respond to S-type defined by Chappell). The isotopic composition of oxygen, hydrogen and carbon in Gejiu tin de- posite shows that the tin mineralization occurred mainly in the deuteric thermal fluid stage.The ore-bearing fluids were mainly derived from the magmatic wa- ter, and a large of atmospheric water joined to it only in its later stage. Some carbons in the ore-bearing fluid was come from the organic carbon resulted from marblization of the surrounding rocks.