A facile method has been developed to encapsulate Fe3O4 nanocrystals (NCs) in morphology-tunable fibers (belt-like, solid, and tubal) by using a sonochemistry driven synthesis and a subsequent reflux procedure. By adapting the use of tetraethyl orthosilicate, ammonia, Cd2+, and thiolglycolic acid (TGA) to an ultrasound-driven synthesis, the Fe3O4 NCs were coated with a thin composite shell. Supersonic treatment plays an important role to prevent the agglomeration of the Fe3O4 NCs in an alkaline condition. The composite shell became thicker due to the deposition of SiO2 monomers, Cd-TGA clusters, Cd2+, and free TGA molecules during reflux. In addition, these composite shell-coated Fe3O4 NCs were assembled in composite fibers which were created by the growth of Cd-TGA clusters and the deposition of SiO2 monomers. The Fe3O4 NCs mono-dispersed in fibers revealed superparamagnetic behavior. The magnetic saturation value of tubal fibers is lower than those of belt-like and solid fibers. These fibers with Fe3O4 NCs would be utilizable for further application. The strategy described here should give a useful enlightenment for the design and fabrication of morphology-tunable fibers with functional NCs. (C) 2010 Elsevier Inc. All rights reserved.