Traditional methods and instruments are not able to measure the true Root Mean Square(RMS) of distortion sine wave or non-sine signal accurately. This article introduces a method to measure the true RMS of any high frequency signal by using the true RMS measurement IC，AD536A，designed by AD corporation. The paper analyses the difference between the RMS measured by the average measure method and the true RMS in theory. It explains the disadvantages of current RMS measure method，and presents the design of true RMS measure circuit. The experiment results prove that the proposed method and circuit can measure the true RMS of any high frequency signal accurately.