We report on the realization of a displacement sensor based on an optical nanofiber. A single gold nano-sphere is deposited on top of a nanofiber and the system is placed within a standing wave which serves as a position ruler. Scattered light collected within the guided mode of the fiber gives a direct measurement of the nanofiber displacement. We calibrated our device and found a sensitivity up to 1.2 nm/Hz—{\surd}. As an example of application, a mechanical model based on the Mie scattering theory is then used to evaluate the optically induced force on the nanofiber by an external laser and its displacement. With our sensing system, we demonstrate that an external force of 1 pN applied at the nanofiber waist can be detected.