Abstract: A toughener that can effectively improve the interlaminar toughness in carbon fiber composites is crucial for various applications. We investigated, the toughening effects of phenolphthalein-based cardo poly (ether sulfone) (PES-C) on E51/ DETDA epoxy and its carbon fiber composites (CFCs). Scanning electron microscopy showed that the phase structures of PES-C/epoxy blends change from island (of dispersed phase) structures to bi-continuous structures (of the matrix) as the PES-C content increased, which is associated with reaction-induced phase separation. After adding 15 phr PES-C, the glass transition temperature (T_g) of the blends increased by 51.5 °C, and the flexural strength, impact strength and fracture toughness of the blends were improved by 41.1%, 186.2% and 42.7%, respectively. These improvements could be attributed to the phase separation structure of the PES-C/epoxy system. A PES-C film was used to improve the mode-II fracture toughness (G_IIC) of CFCs. TheG_IIC value of the 7 μm PES-C film toughened laminate was improved by 80.3% compared to that of the control laminate. The increase inG_IIC was attributed to cohesive failure and plastic deformation in the interleaving region.