Mono-sized and single-walled carbon nanotubes (SWNTs), synthesized by pyrolyzing tripropylamine molecules in the channels of porous zeolite single crystals, have been directly observed by high-resolution transmission electron microscopy (HRTEM). As confirmed by imaging simulations, SWNTs measure just 4 Angstrom in diameter. HRTEM images of these tiny SWNTs show simple contrast of paired fringes. The spacing and shape of the paired fringes are sensitive to the focus condition of HRTEM. When imaging is done at the Scherzer defocus condition, the spacings between the paired dark fringes (outside-to-outside) represent the actual diameters of SWNTs.