Ultra-small single-walled carbon nanotubes (SWNTs) with diameter of 0.4 nm were fabricated in the channels of AlPO -5 crystals by pyrolyzing hydrocarbon molecules. In order to improve the structural quality of the SWNTs, we introduced Brønsted acid sites onto the channel walls by incorporating metal cations (Mn, Mg, Co, and Si) into AlPO -5 framework. The Brønsted acid sites play an important catalytic role in the carbonization of hydrocarbon molecules (tripropylamine) in the AlPO -5 channels, and favor the formation of SWNTs, as revealed by the significant decrease in formation energy of the nanotubes. The experimental results agree well with the predictions of first-principles calculations. © Springer Science + Business Media, LLC 2006.