The epoxy matrix in carbon fiber/epoxy composites was modified with graphite nanoplatelets to improve the matrix dominated mechanical properties of the composite. A prepreg-autoclave process was developed for preparation of nanoparticle-reinforced fiber composites. The in-plane shear modulus and longitudinal compressive strength were enhanced. The compressive strength of the composite was increased by 43 % and 44 %, and the in-plane shear modulus was improved by 7 % and 15 % for 3 wt% and 5 wt% of nanoparticle loadings, respectively. This mechanical enhancement is mainly attributed to the reinforcement of matrix phase by the nanoparticles. However, the substantially improved compressive strength is attributed in large part to the reduced waviness of the fibers in the uni-weave perform caused by the nanoparticles between layers.