The crystal structure and defects in a commercial TWS-100 beta-SiC whisker were observed and analyzed by means of high-resolution transmission electron microscopy. It was found that the SiC whiskers with triangular or hexagonal cross-sections both have a face-centered cubic structure, while their crystal defects are different. The defects in the triangular whisker are mainly stacking faults on the (111) planes which are not perpendicular to the whisker axis, however, in the hexagonal whisker there are a great amount of micro-twins and stacking faults on the (111) planes perpendicular to the whisker axis. The high-density defects account for the hexagonal close-packed (HCP) diffraction patter obtained in the HCP beta-SiC whiskers, which is first pointed out by analyzing the crystal structure of the beta-SiC whisker. (C) 2003 Elsevier B.V. All rights reserved.