Abstract: With the analysis method of the critical fragment length of a single fiber, the interfacial bonding of glass fiber (GF) with polypropylene (PP) is researched. It is found that the interfacial shear strength(ISS) of PP matrix with untreated GF only amounts to 2.75 MPa. However, on the basis of pretreatment of glass fibers with suitable coupling agent,if the fibers are further treated with bismaleimide(BMI), ISS of the composite can be enhanced to 4.42 MPa (increasing by 60%). By blending PP matrix with the anhydride grafted PP (MPP), ISS of the composite can be enhanced to 9.20 MPa (increasing by 233.9 %). With the analysis method,not only can it be accurately determined,whether the interfacial bonding of PP with GF is stronger or weaker,but also can the reliable basis be provided for the design of the final composite.