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.