Here we developed an innovative flow sensor using ultrathin (i.e., 4 mu m) glass sheet fabricated by femtosecond laser processing. The sensor can quantify the flow velocity of liquid flows by measuring the displacement and vibration frequency of the ultrathin glass sheet induced by flows in the microchannel. The developed flow sensor is transparent, chemically inert, and can measure water flows with a velocity between 0.067 and 0.804 m/s (Reynolds numbers 3-36). The displacement of the ultrathin glass sensor varying from 0.4 to 3.1 mu m has negligible influence on the fluid conditions in themicrochannel. Moreover, the sensitivity and dynamic range of the sensor can be readily adjusted by optimizing geometric parameters, such as the aspect ratio and thickness of the ultrathin glass sheet. Besides, the sensor is capable of multiposition measurement to investigate the differences in localized flow velocity within the microchannel. Compared with existing cantilever-based flow velocity sensors, our developed sensor has a higher sensitivity [409 mV/(m/s)], a larger measurement range (0.067- 0.804 m/s), and a smaller displacement range (0.4-3.1 mu m), which is desirable for high-throughputmicrofluidics-based applications, such as cell separation and cell-based therapeutics.