Provided is a method for measuring fluid's viscosity and mass flow, including the steps of immersing one end of an optical fiber into fluid, emitting a light beam by a light source, receiving the light beam by the optical fiber; and measuring the viscosity and the mass flow of the fluid in a sink based on a displacement of the one end of the optical fiber. The method further includes tapering the one end of the optical fiber and coating a surface of the one end of the optical fiber with a deposited gold thin film as a mirror. The light source is an unpolarized diode laser. Accordingly, the size of the measurement system setup is reduced by using ultra-small sensors. Moreover, the present disclosure also provides methods for fluid's viscosity and mass flow measurement by using intensity modulating sensors, elliptical core two-mode fiber-optic sensors and Fabry-Perot interferometer sensors.