The combined analysis of the fracture toughness enhancement of carbon nanotube (CNT)-reinforced composites is herein carried out on the basis of atomistic simulation, shear-lag theory and facture mechanics.It is found that neither longer reinforced CNTs nor stronger CNT/matrix interfaces can definitely lead to the better fracture toughness of these composites.In contrast, the optimal interfacial chemical bond density and the optimal CNT length are those making the failure mode just in the transition from CNT pull-out to CNT break.