The effect of carbon nanoreinforcements of different shapes on the mechanical properties of epoxy-based composites is studied. It is found that while nanodiamond and fibrous (carbon nanotube and nanofiber) particles provided better tensile properties, platelet (graphene oxide) nanoreinforcements lead to a considerable increase in the fracture toughness of the composites. The trend of the results is explained on the basis of the geometrical characteristics of the reinforcements. The accuracy of several micromechanics-based criteria for predicting the Young's modulus of composites is investigated for different nanoparticle shapes. The state of dispersion of nanofillers and the fracture surface features of all composites are examined using TEM and SEM. The effects of carbon nanoreinforcements of different shapes including spherical (ND) and cylindrical fibers (CNT and CNF), and platelets (GO) on the mechanical and fracture properties of epoxy-based nanocomposites are studied. The ND nanoparticles show higher improvements in Young's modulus and tensile strength, whereas GO platelets lead to a higher enhancement of fracture toughness.