This paper presents a new symmetrical flow route of perpendicular rupturing to realize the controllable preparation of monodisperse O/W and W/O emulsions by using a cross-junction microfluidic device. Uniform plugs ranging from 300 to 1800 pm were successfully prepared. The effect of surfactants on two-phase flow performance, two-phase flow patterns under different continuous phase and dispersed phase flow, and the influences of flow rates and the viscosity of continuous phase on plug length were studied. The formation mechanisms of plug flow have been discussed. Considering both the equilibrium between shear force of the continuous flow and interfacial tension, the wetting property between dispersed phase and walls of microchannel, and the influence of oil/water flow ratio on the shape of the interface, a quantitative equation was developed to predict the plug length, which is consistent with the experimental results. (C) 2007 Elsevier B.V. All rights reserved.