To establish an optimization methodology for the emulsification process in microchannels, we have investigated the relationship between the channel geometry of micromixers and the size of the formed droplets. We focus the channel geometry of orifice including the sudden contraction and expansion of the flow. The experimental results indicate that the channel geometry is effective in producing fine droplets, and that the mean droplet diameter is predicted on the basis of the pressure drop due to convection and the energy dissipation rate in the mixer chamber irrespective of the orifice geometry. In the orifice-shaped micromixer, the kinetic energy given to a fluid by contraction is dissipated within the order of milliseconds by the formation of a jet flow of the order of several hundreds of micrometers; we have named this jet "micro-jet.'' On the basis of this mechanism, the energy dissipation rate in a micro-jet, epsilon(jet), is quantified. The mean droplet diameter is proportional to epsilon(-0.4)(jet) irrespective of the channel geometry of the mixer. Using this formulation, we have integrated the geometric parameters of the orifice and the physical properties of the fluids into the expression of epsilon(jet) to establish an optimization methodology of the droplet formation in the orifice-shaped micromixer. (C) 2010 Elsevier Ltd. All rights reserved.