ABSTRACTThe effects of novolac resin (NR) and modified novolac resin (MNR) on the stiffness and toughness of the carbon fiber (CF)/epoxy composites were studied when using these two kinds of modifiers with different physical and chemical performance. The results show that both NR and MNR are excellent toughening agents for the CF/epoxy composites, and the composites treated with MNR present better toughness than that of NR. However, the stiffness of the composites is almost unchanged when adding MNR. The mechanisms were analyzed by fracture studies using scanning electron microscopy and the decrease of cross‐link density caused by MNR was deduced to the toughening effects.