Ultra-small single-walled carbon nanotubes (SWNTs) were synthesized in the matrix of AlPO4-5 zeolite single crystals. Polarized and temperature-dependent micro-Raman spectra were investigated for this complex. Three dominant peaks were exhibited in the spectra, which could be related to the possible three structures of 0.4 nm SWNTs: the (5,0), (4,2), and (3,3). Due to the large curvature in such SWNTs, the sigma and pi unoccupied orbital have a mixing. This orbital rehybridization induced obvious softening in the radial breathing modes and smaller temperature-dependence of Raman active modes compared with tubes with larger diameters. (C) 2003 Elsevier B.V. All rights reserved.