Functional polyethersulfone (PES)/carbon black (CB) films were fabricated and interleaved into carbon fiber/epoxy composite laminates to simultaneously improve the mode I interlaminar fracture toughness (G IC ) and delamination monitoring sensitivity. It was found that when the CB content in the functional film was 7.5 wt%, the initial and propagation G IC of the laminate achieved the greatest improvements of 99.16% and 82.83%, respectively. The toughening mechanism was mainly attributed to the formation of phase separation structure, the plastic deformation of PES and the pull-out of epoxy and CBs. Meanwhile, a delamination increment-resistance change relationship equation was proposed to predict the delamination increment of the laminate during mode I delamination. Moreover, the effect of CB content in the film on the delamination monitoring sensitivity was investigated. The pure PES film showed the largest sensitivity coefficient k of 4.55, which was much higher than that of Control (0.14). When the CB content was in the range of 2.5-7.5 wt%, both G IC and k remained at relatively high levels, indicating that PES/CB films have promising applications in advanced multifunctional composites due to their low cost, simple manufacturing process, and largescale industrial manufacturability.