This article presents design and fabrication of a biosensor based on tapered fiber by measuring the output spectrum in the presence of liquids with different refractive indices. The fiber taper was made by oxy-butane torch. The spectrum is obtained using a broadband light source and an optical spectrum analyzer (OSA), in the wavelength range of 40nm. The effect of changing the refractive index on output spectrum was investigated. It is shown that by increasing the refractive index of the surrounding environment, the output spectrum is shifted and a sensitivity of the order of 0.1 nm/RIU is obtained.