Hypothetic silicon nanotubes are studied using density functional theory calculation based on B3LYP/6-31G(d). Two kinds of precise structures, including armchair and zigzag silicon nanotubes, are first obtained. The structural and electronic properties are discussed in detail. Our result also predicts that all the small diameter silicon nanotubes are metallic regardless of their chiralities. Compared with zigzag nanotubes, armchair nanotubes of silicon are the most reasonable structures due to the efficient overlapping of p orbitals and delocalized pi bonds. With the development of chemical synthesis, armchair nanotubes are hopeful to be synthesized first in the future. (C) 2003 Elsevier B.V. All rights reserved.