Optical properties of single-wall carbon nanotubes (SWCNTs) have been extensively studied during the last decade, and much basic knowledge has been accumulated on how light emission, scattering, and absorption occur in the realm of linear optics. However, their nonlinear optical properties remain largely unexplored. Here, we have observed strong third harmonic generation from highly aligned SWCNTs with intense mid-infrared radiation. Through power dependent experiments, we have determined the absolute value of the third-order nonlinear optical susceptibility, ‡ (3) , of our SWCNT film to be 6.92 × 10 -12 esu, which is three orders of magnitude larger than that of the fused silica reference sample we used. Furthermore, through polarization-dependent third harmonic generation experiments, all the nonzero tensor elements of ‡ (3) have also been extracted. The contribution of the weaker tensor elements to the overall ‡ (3) signal has been calculated to be approximately 1/6 of that of the dominant [Special characters omitted.] component. These results open up new possibilities for application of carbon nanotubes in optoelectronics.