Defects of polymer-derived Si-C-O fibers were intensively studied by the SEM and TEM techniques and their originations were also discussed on the basis of factors experiments. The defects were found mainlyin the form of strumaes , pits and splits on surfaces as well as microflaw networks, porosity clusters and inclusionsin the bulk. Factors experiments reveal that a nonuniform or an insufficient curing would result in larger-sized stru-maes or interior microflaws. Gas evolution rates due to different firing rates have a great influence on the formationof internal microfiaws or porosity clusters and some oxidation- induced pits or splits may be formed on surfaces be-cause ora trace of oxygen or water vapor accumulated from the flowing inert atmosphere during pyrolysis.