The metamorphism of marble of Banyukou Formation in Wutaishan region, Shanxi province, is controlled by buffering process according to its total mineral assemblage of tremolite, phlogopite, dolomite, calcite, microclineane, quartz.The Xco2 of fluid in epidote vein in the Kouquan valley is 0.08; while in its surrounding clolostone, larger than 0.4. The difference in Xco2 in the vein and its surrournding rock also provides evidence that the fluid composition is controlled by buffering.The fluid composition in epidote vein remains unchanged, it shows that fluid composition is controlled by infiltration, The mineral assemblage in country rock is phlogopite, tremolite, calcite, and dolomite, which furthermore indicates that fluid composition is controlled by infilatration, To sum up, the metamorphic fluid composition in this region is controlled by combined buffering and iaLfiltration. The infiltrated fluid is usually less than 1/4 rock volume, Where is monomi- neral vein, there is more abundant infiltrated fluid, which in epidote vein is as high as whole rock volume ( 99%).