A simple, compact, and robust refractive index sensor is developed, by twisting a pair of silica microfibers to form a coupling device. The transmission spectrum of the device is highly sensitive to the surrounding refractive index and the highest sensitivity obtained is -2377 nm/refractive index unit (RIU), at the refractive index value of - 1.3680, for the fibers with diameter of ~4.2 μm, and the sensitivity can be further increased to - 2735 nm/RIU for a smaller fiber diameter of ~2.8 μm. Such a device is easy to construct, of low cost, and compatible with the optical fiber system.