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This paper describes a novel microfluidic method to prepare monodispersed water-in-oil (W/O) microspheres utilizing a piezoelectric actuator. In the main channel, a dispersed phase stream becomes focused and stabilized. In the side channels, oscillating streams are generated by the piezoelectric actuator and flow to the main channel to pinch off the focused stream to form a liquid chopper. By changing the frequencies of the liquid chopper, the size of the W/O microspheres can be successfully controlled in the range of 78.55 - 185.63 μm with very good uniformity (lower than 5% of the coefficient of variation). The present method has the advantages over conventional methods such as hydrodynamic flow focusing and T-junction. The proposed chopper method presents good sphericity, no satellite droplets, and size control of the microspheres.