We report a structural transformation from zigzag to armchair in ultrathin SiC nanotubes under uniaxial compression. Our density functional theory calculations also reveal that an unusual type of fourfold-coordinated tubes corresponds to a distinct minimum on the transformation path and these tubes are all semiconducting with diameter-dependent indirect band gaps. Within the diameter range considered, the electronic properties of these three types of SiC nanotubes ( zigzag, armchair, fourfold-coordinated) exhibit distinctly different behaviors. (C) 2008 Elsevier B.V. All rights reserved.