The relationships between crack propagation resistant factor K10，the compositions of PMMA organic glasses，and cross-head rates of clamps were determined by conduc-ting tensile testing of centrical notched samples．It was observed by experiments that influence of plastieizers and cross-linlring agents on crack propagation resistant pro-perties of organic glasses may be explained by molecular motion which influences plastic yielding process near crack tip．The two different crack propagation processes of organic glasses，i.e．slow and fast crack propagation processes，were studied．By studying the morphology，the refractive index，the thickness of fracture surface layers and the relationships of K1c values with cross-head rates of clamps，we are convinced that a "tearing cloth" fracture model for slow crack propagation and secondary fracture model for fast crack propagation of organic glasses may be used to explain all phenomenons which were observed in this experiments．It is interested to observe that every secondary fracture shower on banded structure surface is corresponding to one parabola．