Background: Size and Size distribution of polymeric nanoparticles have important effect on their properties for pharmaceutical application. In this study, Chitosan nanoparticles were prepared by electrospray method (electrohydrodynamic atomization) and parameters that simultaneously affect Size and/or Size distribution of chitosan nanoparticles were optimized.Methods: Effect of formulation/processing three independent formulation/processing parameters, namely concentration, flow rate and applied voltage was investigated on particle Size and Size distribution of generated nanoparticles using a Box–Behnken experimental design.Results: All the studied factors showed important effects on average Size and Size distribution of nanoparticles. A decrease in Size and Size distribution was obtainable with decreasing flow rate and concentration and increasing applied voltage. Eventually, a sample with minimum Size and polydispersity was obtained with polymer concentration, flow rate and applied voltage values of 0.5 %w/v, 0.05 ml/hr and 15 kV, respectively. The experimentally prepared nanoparticles, expected having lowest Size and Size distribution values had a Size of 105 nm, Size distribution of 36 and Zeta potential of 59.3 mV.Conclusion: Results showed that optimum condition for production of chitosan nanoparticles with the minimum Size and narrow Size distribution was a minimum value for flow rate and highest value for applied voltage along with an optimum chitosan concentration.