Developing novel methods to detect and enhance the dispersibility of graphene nanofillers is critical for overcoming the limitations of polybenzoxazine (PBZ) resins, including difficulties in processing, the requirement of large amounts of solvents during sample preparation, and low fracture toughness. In this study, an epoxy resin was used to replace the solvents in PBZ preparation. Methylene-diphenyl-diisocyanate-grafted graphene nanosheets or multi-walled carbon nanotubes were incorporated to improve the mechanical strength and torsional fatigue life of carbon-fiber-reinforced polymer (CFRP) based on the PBZ resin. A novel thin-film image binarization method was developed to easily measure and assess the dispersibility of nanofillers in the nanocomposites at the macroscale. The effect of nanofiller dispersibility on the mechanical properties was systemically discussed. The results indicated that the dispersibility of the nanofillers and mechanical properties can be significantly improved by (1) taking advantage of the low viscosity and high toughness of Epoxy and (2) adjusting the sequence of sample preparation based on changes in the rheological behavior. These improvements ultimately extend the torsional fatigue life of CFRP and thus its applicability.