Abstract: The fiber/matrix interface plays an important role in controlling the mechanical performance of composites.This paper describes a new method for determining the in situ fiber/matrix bond properties of realistic composites.An in situ interfacial strength testing (ISIST) instrument based on the precise position,auto-loading and data acquisition system has been established.In the test,a compressive load is applied axially to the end of individual fibers in a small specimen cut from the real composite,until interfacial microdebonding occurs.The interfacial shear strength of composites can be obtained by using the debonding force through a finite-element analysis.Several factors which affect the analysis of interfacial shear stress are investigated,and a modified micromechanic model of tested fiber,surrounding matrix and surrounding fibers is given.Based on the ISIST technique,this paper investigates the interfacial bond properties between carbon fiber and PMR-15 polyimide matrix for actual composites,and the influences of different treatment methods of carbon fiber surface on the interfacial shear strengh of CF/PMR-15 composites are discussed.Finally,the interfacial debonding mechanism is preliminarily studied combined with TEM technique.