Abstract: A new expression and its mathematical calculation for estimating δ~(18)Ovalue in water by using CO_2-H_2O equilibrium technique have been proposed.In contrast to Craig's original expression, the new one shows δ~(18)O SMOWvalue in water sample directly. Using this expression the mass spectrometricmeasurement and calculation could been simplified without fractionating factorα and the calculation of (α+ρ)_/ρ value is omitted.