The effect of processing variables on the size of chitosan nanoparticles produced by electrohydrodynamic atomization was investigated. The size was strongly affected by process variables such as surface tension, conductivity and flow rate of chitosan solution, applied voltage and distance between the electrodes. A simple scaling law to quantitatively correlate the droplet size and process variables was developed by dimensional analysis and used to predict the diameter of particles produced under a given set of conditions. The model predictions agreed reasonably well with the experimental results.