Ultramicrotomy is being used routinely as a sample preparation technique for transmission electron microscopy (TEM). TEM study of thin sections of a diketone-bis-benzocyclobutene composite reinforced with Celion (trademark of BASF Co.) carbon fibre revealed two types of periodic crack in the fibre. Coarse cracks were due to bending at knife tip in the early stage of the ultramicrotomy. Also, very fine cracks were observed near the fibre-matrix interface and believed to have been induced by the shear lags between the fibre and the matrix. A simple analysis indicated that the coarse and fine crack spacings could be used to obtain the compressive strength of the fibre and the ultimate shear stress at the fibre-matrix interface, respectively. The combination of ultramicrotomy and TEM provides a useful tool to explore the mechanical properties of a composite material on the submicrometre scale, in addition to the other microstructural and compositional information accessible by TEM.