This work aimed at controlling the size of chitosan nanoparticles that were prepared by the ionotropic gelation method. Quantities of chitosan and sodium tripolyphosphate (STPP) were optimized to obtain a fine size and narrow size distribution of the nanoparticles. X-ray diffraction and selected area electron diffraction patterns indicated the existence of both crystalline and amorphous phases of chitosan. Particle size and size distribution were determined using transmission electron microscopy and dynamic laser scattering techniques. The size of the chitosan particles decreased with a reduction in STPP content due to a decrease in cross-link bonding and the formation of aggregate particles. Using this facile approach, the finest particles of chitosan were obtained in a range of 3-6 nm. © 2020 Science Society of Thailand under Royal Patronage. All rights reserved.