A high sensitivity fiberoptic refractive index sensor based on MachZehnder interferometer is proposed and fabricated. The relationship between the resonance wavelength shifts to the refractive index is studied with NaCl and glycerin solution. The experimental results show that the resonance wavelength shifts to longer wavelength with the increase of the refractive index of the surrounding media, and the wavelength shift is nearly linear to the refractive index with a sensitivity is of 4 086 nm/refractiveindex as the refractive index ranges from 1.333 to 1.356. The sensor is easy to fabricate, compact, and will be usefull in chemical and biotechnological industry.