In this work the modelling and simulation of nanoparticle formation according to the technique of nanoprecipitation was done. In this method, the particle is formed due to the further diffusion of solvent into the water, resulting in the aggregation of the associated polymer chains. In order to predict the characteristics of the nanoparticle and also to improve the process, it was developed a mathematical model that considers: (a) the type of polymer; (b) interaction between solvent and polymer; (c) dynamics of solvent diffusion; and, (d) non-homogeneities in the mixing process. The diffusivity between polymer-solvent was modelled by means of the Vrentas & Duda Free Volume Theory, including the Sanchez-Lacombe equation-of-state. The model was written in terms of Partial Differential Equation, and solved with MAPLE for a given initial size distribution. Additionally, it is a moving boundary problem because the diffusion of the solvent out of the droplet leads to its size reduction. Based on a given initial droplet size distribution, the transient behaviour and the final droplet size distribution can be evaluated. The dynamic simulation shows both the evolution of the solvent inside the droplet and the variation of size in time. Additionally, the comparison between experimental and simulated results showed a very good agreement.