A mesh-stacked dielectrophoretic separator was used to selectively capture tungsten carbide particles from a mixture with diatomite particles. An AC voltage with a stable frequency (10 V, 100 kHz) applied to generate a dielectrophoretic force was superimposed on a low-voltage scanned frequency (1 V, 10 Hz-100 kHz) to analyze the impedance between the electrodes. Using this power source, the amount of captured particles and the selectivity of tungsten carbide could be estimated in situ, without the interruption of dielectrophoretic operation for any direct measurement. Simple empirical equations were derived to realize this estimation. It was observed that the weight fraction of the tungsten carbide in the captured particles tended to decrease with the amount of captured particles. Thus, the amount of captured particles should be intensively limited to achieve a targeted selectivity of tungsten carbide with a maximized captured amount. The simultaneous monitoring of the captured amount and the extent of the selectivity realized by the proposed method will be useful to determine an appropriate timing to terminate the separation operation.