Abstract: On the basis of the analysis method of the critical fragment length of single fiber, the interfacial shear strength (ISS) of polypropylene (PP) / glass fiber (GF) composites is researched. It is found that adjusting the dosage of anhydride grafted polypropylene (MPP) can effectively change ISS. The interfacial bonding of the composite does not come up to a steady level until the contacting time of the melted PP with the fiber is longer than 810 min. When ISS is smaller than 4 MPa, the interfacial bonding is too weak and the fibers at both sides of the broken point are debonding with the matrix, forming a hollow tube. However, when ISS is higher than 7 MPa, the interfacial bonding is too strong and many cracks form, which lie in the matrix around the broken point of the fiber and in the vertical direction from the fiber axis. The composite becomes brittle. While ISS is among 4 and 7, the interfacial bonding lies in an optimum situation. Not only the debonding at the interface does not generate, but also these crack s ex tending in to mat rix do no t fo rm. The p ropert ies of macro scop ic mechan ics of the compo sites co llaterally conf irm th is argumen t.