This paper investigates the effect of heat treatment temperature on the interfacial shear strength (IFSS) of carbon/carbon composites reinforced with polyacrylonitrile-based fibers. A series of single fiber push-out tests were performed on specimens heat treated at 1800, 2100, and 2400 A degrees C, using a nanoindenter with a flat ended conical tip. The microstructure was characterized using polarized light and transmission electron microscopy and the debonded fiber/matrix interface was examined using scanning electron microscopy. Wavelet analysis of the load-displacement data was used as an additional tool to investigate the initiation and progression of debonding. Compared to 1800 A degrees C, heat treatment at 2400 A degrees C was associated with a decrease in IFSS, from 12 to 7 MPa. Transmission electron microscopy study showed that the microstructure of the fiber/matrix interphase remained amorphous even with heat treatment at 2400 A degrees C. The decrease in the IFSS can be partly attributed to the reorganization of the graphene sheets in the matrix in the vicinity of fiber/matrix interphase. The thermal expansion mismatch between fiber and pyrocarbon matrix is another possible reason for the observed decrease in the IFSS.