Highly-filled polymers are composites with high concentrations of added particles. For such composites the loading of the particles exceeds 20 vol. % of the filler. Due to high loading, the interactions between the fillers are important and cannot be neglected. Moreover, the behavior of such composites is influenced by many factors: (i) polymeric binder composition, (ii) particle loading, (iii) particle size distribution, (iv) particle shape, and (v) surface chemistry of the particle. During processing conditions the response of the composite is affected by temperature, pressure, stresses, etc. In order to evaluate the processability of highly-filled polymers, the rheological measurements can be used as efficient method. The results of rheological tests enable the prediction of the response of polymer melt to various factors during the processing. In the present investigation the focus was on highly-filled polymers with up to 60 vol.% of various particles. In order to study the chemical effect, spherical materials with similar particle size distribution but different chemical composition were introduced into a binder system with a fixed composition at different filling grades. The effect of filler materials at various filler loading on the rheological properties was systematically investigated. The results showed, that the type of the particles, used as fillers, influence the rheological properties, especially flowability of highly-filled polymers during processing.