Shanbei Gas Field has becom the natural gas supply base for Beijing which lies in central Ordos Basin,north Shanxi province. The gas reservoirs are the upper ordovician Majiagou group. There are two sets ofgas potential sources for the Shanbei Gas Field, the lower Paleozoic marine carbonates with I-typekerogen and upper Paleozoic terrestrial coal-bearing measures with III-kerogen. Application of carbonisotope analysis, this paper make sure the reservoired gas is the ndxture of oil-formed gas originated fromlower Paleozoic and coal-formed gas originated upper Paleozoic, and the mixture degree in Shanbei GasField has been calculated Which can provide quantitatively the contribution of upper and lower Paleozoicsource rocks to the Shanbei Gas Field respectively. This is important for the further exploration anddevelopment. There are two sets of gas potential sources for the Shanbei Gas Field, the lower Paleozoic marine carbonates with I-type reactor and upper Paleozoic terrestrial coal-bearing measures with III-kerogen. Application of carbonisotope analysis, this paper make sure the reservoired gas is the ndxture of oil-formed gas originated from lower Paleozoic and coal-formed gas originated upper Paleozoic, and the mixture degree in Shanbei GasField has been calculated Which can provide quantitatively the contribution of upper and lower Paleozoicsource rocks to the Shanbei Gas Field respectively. This is important for the further exploration and development.