Nano-emulsions are a class of emulsions with fine droplet size. Nano-emulsions with smaller droplet size can present an aspect similar to microemulsions, but, as fundamental difference, nano-emulsions are not thermodynamically stable, and, because that, their characteristics will depend on preparation method. In the so called low energy methods, fine dispersion is obtained by chemical energy resulting of phase transitions taking place through emulsification path. The adequate phase transitions are produced by varying the composition at constant temperature or by varying the temperature at constant composition, phase inversion temperature method (PIT). In present work, an experimental study about the influence of preparation variables, addition rate and agitation velocity, on nano-emulsions obtained by varying composition method by water addition is carried on. The study is applied to two different systems with water as continuous phase: Brij30/Brij97 as surfactants and isopropylmiyristate as oil; and span20/tween20 as surfactants and liquid pararaffin as oil. Mean diameter of droplets is determined by dynamic light scattering. Preparation is carried out at two different scales, 0.1 a 1.0 L, and in each experiment water addition rate and agitation velocity are fixed following experimental designs. Results are fitted to give mean diameter of nano-emulsion droplets as a function of significant terms of preparation variables. For small scale there is an optimum agitation velocity, while the lower the addition rate the smaller droplet diameter. For 1.0 L scale droplet diameter is smaller than for smalls scale and does not present significant variations with preparation variables.