Insulator-based dielectrophoresis (iDEP) is a dielectrophoretic mode where non-uniform electric fields are created employing insulating structures. This article presents an evaluation of the selectivity of an iDEP microdevice by employing DC electric fields with mixtures of polystyrene nano and microparticles. Experimental and mathematical modeling work was carried out in order to assess the selectivity of an iDEP microdevice to immobilize only the large particles from a binary mixture. Negative dielectrophoretic trapping was observed and the effects of particle concentration, concentration ratio, size ratio, and magnitude of the applied electric potential (200 to 600V) on the microdevice selectivity were studied. The results demonstrated that high selectivity can be obtained with iDEP.