Experimental characterization and theoretical simulations both confirmed single-walled 0.4 nm carbon nanotubes (SWNTs) to be directly observable by high-resolution transmission electron microscopy (HRTEM), with amorphous carbon films as the supporting substrates. While the amorphous carbon film gave rise to a uniform contrast which can overwhelm that of the SWNTs lying flat on the substrate, inclining SWNTs with an angle greater than 30degrees to the supporting film were found to result in a notable contrast of paired dark fringes. This SWNT contrast could be further enhanced by changing the HRTEM defocus condition. (C) 2003 Elsevier Science B.V. All rights reserved.