Monodisperse and size-controlled spherical polymer particles were synthesized by in situ photopolymerizationof O/W monomer emulsions. Monomer droplets were produced without surfactant or pretreatment at a needletip in a simplified axisymmetric microfluidic device. The effect of the viscosity of the continuous phase on theparticle size was studied. The system operated in the dripping mode, at a low Reynolds number. A dimensionlessmaster curve describes the particle diameter as a function of the needle inner diameter as well as velocity andviscosity ratios of continuous and dispersed phases. An empirical law predicts the particle size. The normalized particlediameter depends upon the ratio of the capillary numbers of continuous and dispersed phases with an exponent equalto −0.22.