This paper presents the measurement of ferrite/austenite phase fraction using amulti-frequency electromagnetic sensor. A simple analytical model was established that can describe the response of thesensor for samples containing varying fractions of ferromagnetic phase over a wide range of frequencies(100Hz-1MHz).In particular, a new feature, the peak frequency of the imaginary part of the inductance, is found to be able to distinguish between samples across the whole range of the ferrite percentages. FEM models were used to simulate representative real microstructures from the samples and to relate the relative permeability to the ferrite fraction. Experimental results suggest that the accuracy of ferrite/austenite percentage measurement is within8%.