A comprehensive study was made on the structure of epitaxial thin films of C60 and C70 by means of transmission electron microscopy. Both the films show similar face-centered cubic structure and are epitaxial on (001) mica with close-packed plane parallel to the substrate surface. Two main kinds of defects-stacking faults and twins-were observed and are discussed. The effect of the remaining C70 impurity on the crystal orientation of C60 films was studied by comparing different samples made from high-purity fullerene and C60/C70 mixtures. The results show that there is a higher density of planar defects in the films containing larger amounts of impurities : moreover, some faint anomalous reflections located at so-called 2a0 fcc reciprocal lattice points were also detected, probably as a result of C70 contamination. Finally, it is found that stacking disorders can be easily increased by keeping the high-quality pure C60 film in air at room temperature for a few weeks, implying the instability of the crystal orientation of the epitaxial fullerene films.