The effect of the orientation of carbon fillers with different aspect ratios on the resistivity and morphology of conductive polymer composites (CPCs) based on polypropylene was investigated in this study. Multiwall carbon nanotubes (MWNTs) and carbon black (CB) were used as conductive fillers. The CPCs were made by melt compounding, hot pressing, and solid-state drawing. The alignment of the filler was observed after solid-state drawing. The resistivity of the composites increased with the draw ratio at relatively low carbon filler loadings (<20 wt %), whereas it remained unchanged at a high filler loading (20 wt %, CB). Orientation-promoted anisotropy of the conductive network was observed in both the morphology and resistivity. MWNTs were found to be better at maintaining a percolating network under large deformations than CB because of their larger aspect ratio and their entangled network structure. The experimentally obtained resistivity was analyzed with percolation theory, and this indicated that the initial three-dimensional conductive network was deformed into a two-dimensional network after solid-state drawing for the composites containing CB. The three-dimensional network was found in isotropic CPCs containing MWNTs with the same analysis. Theoretical analysis using excluded volume theory was in good agreement with results obtained experimentally. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 742-751, 2009