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Three types of core-shell particles with different epoxy contents were used as a toughening agent for bisphenol A-derived epoxy resins and carbon fiber-reinforced plastics (CFRPs) prepared therefrom. Influences of the epoxy content of the core-shell particles on heat resistance, rigidity and toughness of the cured epoxy resins and the CFRPs were investigated. The addition of the epoxy-containing core-shell particles slightly increased glass transition temperature (Tg) of the cured epoxy resins. An increase in the epoxy content of the core-shell particles increased tensile modulus (E) of the cured epoxy resin among the samples in the presence of the core-shell particles. The addition of the core-shell particles decreased the interfacial shear strength (IFSS) between the cured epoxy droplet and a carbon fiber in the measurement by the micro-droplet method, but an increase in the epoxy content of the core-shell particles increased IFSS. In the single-edge-notch bending (SENB) test, the presence of the core-shell particles improved the stress intensity factor (KIC) of the cured epoxy resins, but an increase in the epoxy content of the core-shell particles rather lowered KIC of the cured epoxy resins. An increase in the epoxy content of the core-shell particles increased E of the prepared CFRPs among the samples containing the core-shell particles. The presence of the core-shell particles improved the Charpy impact strength (αCN) of the cured epoxy resins and the CFRPs, but an increase in the epoxy content of the core-shell particles did not necessarily increase their αCN. In the end notched fracture (ENF) test, interlaminar fracture toughness (G) of the CFRPs was improved by the presence of the epoxy-containing core-shell particles, but an increase in the epoxy content of the core-shell particles rather lowered G of the CFRPs. These results indicate that the use of optimum epoxy content of the core-shell particle is important to obtain highly-tough CFRPs.