Abstract: The sensitivity of long-period grating in refractive index sensing was analyzed by using the coupled-mode theory, and the formula of resonance wavelength shifts of LPFGs which changes relative to the surrounding refractive index was derived. If the period of grating is not changed, when the cladding refractive index is less than and close to the surrounding refractive index, the results show that the resonant wavelength shifts increased. The resonant wavelength shifts are greater for higher order cladding modes and smaller fiber cladding radius and smaller cladding refractive index, namely, the sensitivity to the surrounding refractive index is obviously enhanced. When the surrounding refractive index is greater than the fiber cladding refractive index, the resonant wavelength of grating will remain unchanged.