This investigation gives attention on the rheology characteristics of polymer composites based on graphite and carbon black as fillers for further processing using an injection molding machine. In such a high solid loading system, the particles exhibit a very strong tendency toward agglomeration. This rapidly increases the viscosity of the mixture and decreases moldability. The presence of agglomerates in the mixture in particular may result in defect within the microstructure in the final product, even though it is sometime necessary in electrical conductivity. Composite materials in this study are polypropylene (PP) as matrix, and graphite (G) and carbon black (CB) as fillers, with a varied composition according to the percentage weight (% wt) of CB. Twin screw co-rotating extruder was used for mixing materials in order to achieve the best homogeneity of this compound. The measurement results obtained using capillary rheometer equipment showed that the addition of CB to the mixture of PP/G increase the viscosity of the materials, increase the activation energy and generally reduce the fluidity of composite materials. The value of the mixture viscosity increases with increasing the number of CB, reducing the ability of materials to be formed (moldability). Material viscosity, activation energy, fluidity and moldability shows how suitable the compound material to be processed by using injection molding machine.