The separation systems based on dielectrophoretic field-flow-fractionation (DEP-FFF) are used for a wide range of bioparticle types, including cells, bacteria, viruses, proteins, etc. An array of interdigitated microelectrodes lining the bottom surface of a thin chamber is used to generate dielectrophoretic forces that levitate the bioparticle mixture. The balance between DEP levitation and gravitational forces determines the bioparticles position at equilibrium heights within a fluid-flow profile, and consequently determines their velocities and the corresponding elution times. The elution time depends on the voltage applied on the microelectrodes, geometry of the device, bioparticle dielectric properties and density. This paper analyses numerically the behavior of a bioparticle mixture suspended in a dense and viscous fluid under dielectrophoresis. The controlled spatial separation of bioparticle mixture is performed by a combination of dielectrophoretic and hydrodynamic forces. The theoretical background and a set of numerical results (calculated DEP force, particle trajectories, etc.) are presented. The numerical solutions are obtained using the COMSOL Multiphysics finite element solver. The presented results demonstrate that the DEP-FFF method can be successfully applicable to many biomedical cell separation problems, including microfluidic-scale diagnosis and preparative-scale purification of cell subpopulations.