Dielectrophoresis is the electrokinetic movement of particles due to polarization effects in the presence of non-uniform electric fields. In insulator-based dielectrophoresis (iDEP) regions of low and high electric field intensity, i.e. non-uniformity of electric field, are produced when the cross-sectional area of a microchannel is decreased by the presence of electrical insulating structures between two electrodes. This technique is increasingly being studied for the manipulation of a wide variety of particles, and novel designs are continuously developed. Despite significant advances in the area, complex mixture separation and sample fractionation continue to be the most important challenges. In this work, a microchannel design is presented for carrying out direct current (DC)-iDEP for the separation of a mixture of particles. The device comprises a main channel, two side channels and two sections of cylindrical posts with different diameters, which will generate different non-uniformities in the electric field on the main channel, designed for the discrimination and separation of particles of two different sizes. By applying an electric potential of 1000 V, a mixture of 1 and 4 mm polystyrene microspheres were dielectrophoretically separated and concentrated at the same time and then redirected to different outlets. The results obtained here demonstrate that, by carefully designing the device geometry and selecting operating conditions, effective sorting of particle mixtures can be achieved in this type of multi-section DC-iDEP devices.