Abstract: The interlaminar-toughening T300 carbon fiber/epoxy (CF/EP) composites with agglomerate microcapsules were prepared using hot-compaction laminating technology. The toughening effect was evaluated through mode Ⅰ interlaminar fracture tests conducted on double cantilever beam (DCB) CF/EP composites. The interlaminar properties of virgin and toughened CF/EP composites were evaluated by ultrasonic guided wave technology. The microstructures of interlaminar fracture surfaces of CF/EP composites were observed by SEM to reveal the toughening mechanism of microcapsules and explain the ultrasonic guided wave detection results. The results indicate that the microcapsules are distributed in the interlamination matrix resin in the form of agglomeration, which effectively improve the interlaminar fracture toughness of CF/EP composites. The filling of microcapsules changes the characteristics of the interlaminar matrix of the CF/EP composites, which increases attenuation of the propagating guided waves and causes the response signal peak to decrease. Meanwhile, the agglomerate microcapsules change the vibration response of the CF/EP composites to the excitation of the five-peak wave with a center frequency of 125 kHz, resulting in the amplitude of the center frequency in the signal spectrum being lower than that of the virgin CF/EP composites.