In order to accurately determine a small fraction of a ferromagnetic phase in a paramagnetic matrix, the difference in ferromagnetic and paramagnetic responses to an applied magnetic field has been analyzed. The fraction was examined by field-dependent magnetization measurement at different temperatures as well as by thermo-magnetic measurements at constant magnetic fields. The method was successfully applied for the determination of small martensite fractions in a cold-rolled twinning-induced plasticity austenitic steel at three different thickness reductions.