The microstructure of diamond-SiC interfaces was studied by transmission electron microscopy (TEM). Specimens were prepared by focused ion beam (FIB) etching from a diamond-SiC composite bulk material. The diamond-SiC interfaces were easily located by high contrast in FIB images of the bulk surface, and site-specific specimen preparation was possible. The possible origin of this high contrast in FIB images compared to SEM images is discussed. TEM images and electron diffraction patterns showed that the diamond and SiC crystals away from the interface region are relatively defect-free, but numerous defects are present at the diamond-SiC interface over a dimension of 600 nm, much larger than the physical interface.