Fiber reinforced polymer composites are two component material systems in which fibers are embedded in a polymer matrix. Such a system inherently has an interface where the two components meet. Adjacent to the interface extending beyond the fiber surface is the “interphase region.” Properties within the interphase vary due to variations in the chemistry. The study of mechanical property variations with changing chemistry will help in better understanding and tailoring of the composite properties. The present work concentrates on the investigation of nanomechanical properties within the interphase of a glass fiber embedded in polyester matrix system. The glass fibers were coated with two types of silanes to produce a strong and a weak bond at the fiber-matrix interface. Nanoindentation techniques coupled with atomic force microscopy imaging capabilities have been used for this investigation. Two different tips were employed for indenting, one being a Berkovich diamond tip supplied by Hysitron, Inc., Minneapolis, MN and another being a parabolic tungsten tip, which was made in the laboratory. Indentations were performed within the interphase region, also in the bulk matrix, and on the glass fiber. The variation in mechanical properties such as modulus, stiffness, hardness, and penetration depth were obtained within the interphase by indenting at the fiber surface outward. Variations of the elastic modulus in the interphase region and its relation to the chemistry are presented. The results obtained using two different tip shapes have been compared. Phase imaging was performed using tapping mode atomic force microscopy to qualitatively identify the presence of an interphase near the glass fiber-polyester interface. These experiments show that when no coupling agent is used the interphase thickness is less than , and its exact determination is limited by the spatial resolution of the tips employed and the process of indentation. Phase imaging results with composite samples made of coated glass fibers corroborate the results obtained from nanoindentation experiments.