In order to improve the application of nano-particles/polymer composite, we investigated the relationship between microstructures and electrical properties on injection molded composites made from carbon black(CB) particles mixed with polypropylene(PP). Standard tensile specimens were injected under different processing conditions, after which, the specimens were cut off along the surface to observe the microstructures at different positions of the moldings. The microstructures were observed with Scanning Electrical Microscope(SEM), and electrical properties were measured by using a two-terminal standard resistor under DC condition at room temperature. The results showed that CB nano-particles may form the best conductive path under the higher packing pressure combined higher injection pressure. If packing pressure is low, the resistivities at the skin zone loaded by high injection pressure are smaller than low injection pressure, but the resistivities increase at the sub-skin zone. We found that the sub-skin zone is a high resistivity district which would expand with higher injection pressure matched lower packing pressure. Contrast to the injection molded polymer whose microstructural orientation is stronger at the sub-skin zones, injection molded CB particles/polymer composites develop strong oriented microstructures at the core zones in stead of the skin or sub-skin zones due to CB particles’ migration.