The size control of gold nanoparticles synthesized in surfactant free water with a continuous flow mode was elucidated and used to produce higher concentration (3 mM) of stabilized gold nanoparticles. The originality of the synthesis was to finely modulate the initial pH of the reducing agent instead of the gold precursor to modify the kinetic of the reaction. The acceleration of the kinetic (~1 s) prevents the modification of the gold precursors ensuring the control of the final size (from 3 to 25 nm) of the nanoparticles with a low polydispersity for aqueous surfactant free solution. The accurate measure of the size distribution by small angle X-ray scattering was combined to the use of a model based on the coupling of nucleation and growth equations together with a progressive injection of monomers. The results on the final state show that the size of the nanoparticles is indeed controlled by the kinetic of reduction of gold atoms. A millifluidic setup equipped with a homemade mixer offers a robust way of rapid mixing to obtain a reproducible production of large amounts of nanoparticles.