Abstract: Carbon fiber/epoxy (CF/EP) composites are widely used in the aerospace industry, but their interlaminar properties and out-of-plane thermal conductivity are poor because of the lack of CF reinforcement in the interlaminar area. We prepared carbon nanotube/polyimide (CNT/PI) composite films and used them to improve the interlaminar fracture toughness (ILFT) and thermal conductivity of laminates of CF/EP and CNT/PI. Interleaving CNT/PI films with unidirectional CF/EP prepregs increased the mode I (the pre-cracked laminate failure is governed by peel forces) and mode II (the crack is propagated by shear stresses) ILFT of CF/EP laminates by 260% and 220%, respectively, which is attributed to the reinforcement effect of the CNTs and the plastic deformation of PI film. In addition, the out-of-plane thermal conductivity of the laminates is improved by introducing CNT/PI films because of their intrinsic high thermal conductivity and the continuous conductive network of CNTs. This toughening method provides an effective strategy for improving the thermal conductivity and mechanical properties of CF/EP laminates simultaneously.