nanofibersUltrasonication abstract A method of top down preparation of chitosan nanoparticles and nanofibers is proposed. Chitin nanofibrils(chitin NFs) were prepared using ultrasonic assisted method from crab shells with an average diameterof 5 nm and the length less than 3 um as analyzed by atomic force microscopy and transmission electronmicroscopy. These chitin nanofibers were used as the precursor material for the preparation of chitosannanoparticles and nanofibers. The degree of deacetylation of these prepared chitosan nanostructureswere found to be approximately 98%. In addition these chitosan nanostructures showed amorphous crys-tallinity. Transmission electron microscopic studies revealed that chitosan nanoparticles were roughlyspherical in nature and had diameters less than 300 nm. These larger particles formed through self-assembly of much smaller 25 nm particles as evidenced by the TEM imaging. The diameter and the lengthof the chitosan nanofibers were found to be less than 100 nm and 3 m respectively. It is envisaged thatdue to the cavitation effect, the deacetylated chitin nanofibers were broken down to small pieces to formseed particles. These seed particles can then be self-assembled to form larger chitosan nanoparticles.