A study on the mechanism for the degraded toughness in nylon 6/clay nanocomposite is explored in this Article. Such a nanocomposite exhibits lower specific essential work of fracture w(e) and specific nonessential work of fracture beta w(p) than its pure nylon 6 counterpart, as revealed by essential work of fracture (EWF) measurements. Furthermore, the molecular orientation in a small region (20 x 20 mu m(2)) ahead of crack tip, obtained from micro-FTIR measurements for the first time, is found to be lower in the nanocomposite during crack initiation and propagation. The decreased molecular orientation, mostly resulted from severe microvoiding at crack tips, is responsible for the reduced specific essential work of fracture w(e). Meanwhile, the molecular orientation around crack tip also indicates that lower plastic deformation occurs in the plastic zone, which is well correlated with decreased specific nonessential work of fracture beta w(p) in the nanocomposite.