A novel millimeter-sized reactor, named millireactor in the present study, is proposed for oil droplet formation aiming at heterogeneous enzymatic hydrolysis. The reactor consists of a circular ditch with a small stirring bar and a straight channel. To obtain optimal operation conditions and a reactor configuration for droplet formation, the stable droplet formation region was explored by mapping analysis, and the relationship between the specific interfacial area of droplets and other physical parameters were investigated by dimension analysis. It was found that the shear stress parameter and interfacial tension played important roles in the droplet formation. Considering parameters such as the length of the stirring bar, rotating speed, diameter of the stirring section, gap width, interfacial tension, viscosity and residence time in the stirring section, a correlation equation including three dimensionless numbers was obtained. The optimal design guide for obtaining high specific area of oil droplets involves increasing the volume of the stirring section and reducing the gap width as narrow as possible. The reactor performance was evaluated comparing the amount of fatty acid produced in the millireactor versus the batch reactor. The productivity of the millireactor was higher than the batch reactor for the first 10 min. (C) 2010 Elsevier B.V. All rights reserved.