This study investigated the interfacial shear strength (IFSS) of graphite/epoxy composites containing functionalized graphene. Epoxy resin together with pristine graphene and functionalized graphene were employed as a matrix in the fiber composites, with graphene loadings in the epoxy matrix of 0.1, 0.3, and 0.5 wt%. The functional groups grafted on the functionalized graphene were characterized through Fourier transform infrared spectra and X-ray photoelectron spectroscopy. The IFSS of the fiber composites were evaluated through microdroplet testing, and the corresponding failure mechanism was examined using scanning electrical microscopy. The experimental results indicated that the IFSS increased with the graphene loadings in the matrix. Furthermore, both functionalized graphene and pristine graphene provided a similar IFSS in the fiber composites. Observation of the failure interface showed that the failure mechanism of the IFSS was dominated by interfacial failure between the fiber and matrix in conjunction with adhesive failure in the matrix. Introducing graphene in the matrix can enhance IFSS by improving the adhesive strength in the interface as well as complicating the crack extension in the matrix.