This letter presents the experimental investigation to increase the sensitivity of an optical fiber refractive index sensor with a multimode interference (MMI) structure. It is confirmed that the interference wavelength can be set in the long-wavelength region by adjusting the sensing-part length. Moreover, it is shown that the fineness of the cores in the input and output fibers is essential to obtain a sharp interference signal. We demonstrate the high sensitivity of an MMI-structured optical fiber sensor by applying it to refractive index measurements of ethanol/water solutions. This refractive index sensor reveals a resolution as fine as 1.8 × 10^-5, which is the highest value so far reported in this type and has one order higher sensitivity than a commercially available Abbe refractometer (∼ 10^-4).