Abstract: There are six equations have been derived for the double standardmethod of accurate determination of isotope ratio in this paper. Severalmethods of measuring isotope ratio with double paths inlet system can bedesigned according to the basis of these equations. The determinate result ofeach method is coincide with the standard deviation (σ=5×10~(-5)). When theisotope ratios are measured with double standard method, the triple pathinlet system have more advantages than double paths. The advantages havebeen described.