Co-flow glass capillary microfluidic devices with injection and collection capillaries of different inner diameters were prepared. Nitrogen gas and ultrapure water were injected into the devices to prepare monodisperse microbubbles. By changing the inner diameters of the injection and collection capillaries, gas pressure, and liquid flow rate, monodisperse microbubbles with a coefficient of variation (CV) of less than 2% were generated in a diameter range of 4 to 70 mu m at a generation rate of 1.7 x10(5) to 1.5 x 10(2) bubbles/s.