A detailed three-dimensional structural analysis of single-walled carbon nanotubes was carried out using a scanning tunneling microscope (STM) operated at room temperature in ambient conditions. On a microscopic scale, the images show tubes condensed in ropes as well as tubes which are separated from each other. For a single-wall nanotube rope, the outer portion is composed of highly oriented nanotubes with nearly uniform diameter and chirality. On separated nanotubes, atomically resolved images show variable chirality ranges between0°–30°,and variable diameter (1–3 nm), with no one type dominant. From STM and scanning tunneling spectroscopy measurements we confirmed the correlation between chirality and the electronic properties, namely the tuning from metallic to semiconducting. We also observed a rectifying behavior correlated with the chiral angle of25°,an important observation for nanodevices application. ©1998 American Institute of Physics.