The cross/lowing rupture technique was first used in a microfluidic device to prepare microbubbles, and successfully prepared monodisperse microbubbles with polydispersity index (a) values of < 2%. The parameters affecting the microbubble-formation process, such as two-phase flow rates, continuous-phase viscosity, surface tension, and surfactants were investigated. The microbubble-formation mechanisms of the cross/lowing rupture technique with those of the techniques of both flow-focusing rupture and geometry-dominated breakup were also compared. It was also found that the bubble size decreased with increasing continuous-phase rate and its viscosity, while independent of surface tension. The different species of surfactants also influenced the microbubble-formation process. Moreover, the bubble-formation mechanism by using the crossfiow rupture technique was different from the techniques of both hydrodynamic flow focusing and geometry-dominated breakup. The microbubble-formation process using the crossflowing rupture technique is controllable. (c) 2006 American Institute of Chemical Engineers