The effect of hybrid carbon fillers of multi-walled carbon nanotubes (CNT) and carbon black (CB) on the electrical and morphological properties of polystyrene (PS) nanocomposites was systematically studied via microinjection molding (μIM). The polymer nanocomposites with three different filler concentrations, namely 3, 5 and 10 wt%, at various weight ratios of CNT/CB (100/0, 30/70, 50/50, 70/30, 0/100) were fabricated by melt blending, followed by μIM under a defined set of processing conditions. To this end, a rectangular mold insert which has three consecutive sections with decreasing thickness along the flow direction was adopted to evaluate the abrupt changes in mold cavity thickness on the properties of subsequent microparts. All microparts were cut at the transition areas to facilitate characterizations. The electrical conductivity (σ) for each section of the microparts was measured across the transverse direction (TD) and along the flow direction (FD), respectively. Results suggested that the FD σ was invariably higher than the TD σ, which indicates a role for orientation of added carbon fillers along flow direction. In addition, the ratio of FD σ over TD σ for the middle section is at least one order of magnitude higher than the thick section, revealing that the alignment of carbon fillers in the flow direction increased with an increase of shearing effect. It was found that high structure CB is more effective than CNT in enhancing the electrical conductivity of subsequent moldings which could be ascribed to the good dispersion of CB particles in the host matrix and their ability to form conductive networks via self-assembly. The morphology observations revealed that there is a shear-induced depletion of CB particles in the shear layer which could be attributed to the marked difference of shear rates between the shear layer and core layer of the microparts. Moreover, the annealing treatment is beneficial to enhance the electrical conductivity of CNT-containing microparts, which is attributed to the restoration of conductive pathways after thermal treatment.