Abstract: A considerable number of studies on fracture toughness of glassy polymers have been done, yet there is still much controversy about the definition of its KIc because of unique features of the polymer fracture behavior. In this paper, an attempt is made to discuss the plane strain fracture toughness KIc of polymethyl methacrylate （PMMA） according to the principles of linear elastic fracture mechanics and considering the intrinsic characteristics of macromolecular motion in polymeric materials. It is reasonable that the stress intensity factor for crack initiation is defined as plane strain fracture toughness KIC, while K （instability） at crack slow growth--fast fracturing transition indicates the aspects of macro-molecular motion of different polymers.Based on fractographic survey, two models of micromechanism of sub-critical fatigue crack propagation under cyclic loading are proposed to explain the fatigue fracture processes of glassy polymers.From the fractographic survey in subcritical crack region, inclusive of mirror and mist regions of fatigue crack propagation（FCP）, it is concluded that occurrence of fatigue striations is dependent on the level of K and independent of the abrupt transition of K. No fatigue striations at low K indicate continuous crack growth. At high K, discontinuous crack growth is illustrated by appearance of fatigue striations on the fracture surface of either mirror or mist region while one striation corres ponds to one loading cycle.