Positive temperature coefficient (PTC) materials based on amorphous polymers usually suffer from low intensity owing to their relatively small volume thermal expansion. Herein, graphene nanoplatelets (GNPs) and carbon fibers (CF) with large aspect ratios were introduced into amorphous poly(methyl methacrylate) (PMMA) via melting mixing followed by hot pressing to improve the PTC characteristics of the conductive composites. The binary PMMA/GNPs composites showed good reproducibility while the PMMA/CF composites were superior in PTC intensity and their switching temperature (positive to negative temperature coefficient) could be tuned by varying the CF concentrations. When the both binary composites were connected in series, their strengths could be fostered and their weaknesses could be circumvented. The ternary composite exhibited a superior PTC intensity of 7.17, outperforming most of the reported PTC materials. These composites have great potential for applications as temperature sensors and current limiting devices.