Hyperbranched polyphenylene nanotubes are fabricated by Diels-Alder reactions of tetraphenylcyclopentadienones in the nanochannels of porous alumina membranes. Nanotubes with different structures and morphologies, and even highly porous carbon nanotubes (see Figure), can be obtained by adjusting the starting materials and the conditions of in-situ polymerization.