Methods and apparatus for fluid flow velocity and flow rate measurement are provided. Fluid velocity is measured in optical method based on fluorescence photobleaching of a fluorescent dye. The invented method and apparatus requires a calibration relation between flow velocity and fluorescence signal and is easy to use. The invented method and apparatus can measure bulk flow velocity and flow rate inline, two and three components of flow velocity vector. It can also measure flow velocity in near wall region using evanescent wave. Since the invented method and apparatus uses molecular dye and calibration relation between flow velocity and fluorescence signal, it has ultra high spatial and temporal resolution. The invented method can be used not only to apparatuses and devices in conventional size, but also to that in MEMS and NEMS.