In this paper, the basic concept of G/T value and the traditional measurement method of G/T value are briefly described. According to the definition of G/T value, a novel G/T value measurement method for active phased array antenna is deduced. The principle of this method is to determine the G/T value by measuring total gain of the active phased array antenna and the normalized noise power of the system output. Finally, the measurement results of G/T value of S-band 19-element array antenna system are given, and the results of measurement and of the theoretical calculation agree fairly closely, which verifies the feasibility of this method.