An asymmetrical twin-core fiber based Michelson interferometer is reported as a refractive index sensor. One core of the twin-core fiber locates at the fiber center and the other core is 26 μ m away from the central core. Part of the cladding of the twin-core fiber over a small length is removed by chemical etching to make the effective refractive index of the fundamental mode of the side core is sensitive to the ambient refractive index. Therefore, the interference spectrum between the central core and the side core shifts with the variation of the ambient refractive index. The sensitivity of such a Michelson interferometer is ~ 270 nm/RIU in the range of 1.34-1.38.