Polymer matrix composites (PMCs) have been increasingly used for high temperature applications in automotive and aerospace industries. Under such conditions, the materials will experience thermomechanical degradation which can cause premature failure of the composite structures. The desire to predict the damage and lifetime of high temperature polymer matrix composites (HTPMCs) and the structural durability for structural applications has been elusive. The success of the multiscale modeling and analysis relies on the experimental capability of properly characterizing the evolution of mechanical behavior of each constituent: matrices, fibers, and interfaces. This paper presents a navel technique, nanoindentation, to identify the interfaces between dissimilar materials and subsequently to evaluate the physical and mechanical properties across the interfaces. It is proposed to use a nanoindenter equipped with small spherical tip, ≺40 nanometers in radius, to indent across the interfaces of dissimilar materials. The proposed method has been validated by conducting a large number of virtual experiments through 3-dimensional finite element simulations, by varying the properties of the two dissimilar materials, including various combinations of modulus (E1/E2), yield strength (σy1/σy2), hardening index (n1/n2), interface sizes (R/T), Poisson's ratio (v), etc. The mechanical properties across the interfaces have been obtained, and a quantitative model for predicting the interface sizes has been established.