The characteristics of differential pressure fluctuations in vertical upward steam - water two ?phase flow under the condition of high temperature and high pressure are studied by using the signal process technique. The fluctuations are analyzed by using the methods of time domain and frequency domain

which is a power spectrum density (PSD) function.The features of the curves of the power spectrum versus frequency are categorized as two kinds. The feature of the power spectrum density function is extracted by means of the Root Mean Square (RMS)

from which an objective flow regime indicator is developed. The experimental result agrees well with the flow regime map proposed by McQuillan & Whal-ley under the pressure of 3.0 MPa. Future studies are required to improve the accuracy of identification.