Abstract: A new method of holdup measurement for water-oil two phase flow is introduced, i.e. ultrasonic diffusion. The method uses the effect of ultrasonic diffusion to measure holdup of discrete phase. Then the discrete phase holdup can be measured by non-packer without changing flow state. Because of the nonlinear effect caused by flow pattern, oil diameter and absorption coefficient etc, it is difficult to establish the measurement model. The paper establishes RBF neural network to predict water holdup, and the measurement precision can be improved.