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 30° 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.