This work describes a novel method for the preparation of monodispersed calcium alginate microbeads in a microfluidic device. A novel microfluidic device comprising of two cross-junction channels and a synthesizing channel was designed by controlling the mixing condition with laminar flow and two-phase droplet flow in microfluidic devices. At the first junction, the side two streams, sodium alginate aqueous solution and calcium chloride aqueous solution, were separated to prevent contact by the injection of deionized water in the middle channel. Then the aqueous mixture solution was induced into monodispersed droplets by a focusing sheath of immiscible oil phase at the second junction. The formed droplets were solidified in situ in the synthesizing channel with the rapid mixing of reactants in the droplets. The size of the Ca-alginate microbeads can be controlled from 50-300 mu m in diameter with a variation of less than 2 %, simply by changing the two-phase flow ratio. The proposed microfluidic platform is capable of generating uniform emulsions and has the advantages of in situ solidification and good control of particle size.