Abstract: When pipeline oil contains gas, the gas will be dispersed in liquid with dispersed phase, which makes the dielectric constant of mixture reduced and results in a low output voltages of instrument.On the basis of the measurement method of void fraction, the demarcating process is introduced.The experiment results explain the effect of void fraction on water cut online measured, that is, when the void fraction and water cut are respectively less than 25% and 5%, the variation of void fraction per 1% will cause a variation in online measurement result up to 0.1%, and the sign of both is reverse.