We present a sensitive nano-optical fiber biosensor made by shaping a fiber to form a taper with a tip size under . A -thick layer of gold is coated around the tapered fiber and a surface plasmon wave is excited near the tip to achieve a sensitivity of the reflective index unit of (% ) in the intensity measurement. A 3-D coded finite-difference time-domain approach verifies the excitation of the surface plasmon wave and the differences among its intensities in media of various refractive indices. The nanotip fiber sensor has the merits of a low background light and an ultrasmall detection region. Only a microliter of sample solution is required for detection.