This study proposes a thinned-cladding zeolite coated long period fiber grating (LPFG) chemical sensor. The cladding outside the grating zone is etched by hydrofluoric (HF) acid, and then the zeolite film is coated on the thinned grating. The sensitivity characteristics of the LPFG to different external refractive indices are investigated. By etching cladding, the resonant wavelength shifts towards the longer wavelength (red shift) while the attenuation band depth is increased. In addition, with very large surface-to-mass ratio, zeolitic pores can efficiently adsorb molecules from the ambient for highly sensitive detections. The adsorbed analyte molecules are collected and aligned in nanoscale zeolite pores. This leads to changes in the refractive index of zeolite coating, which in turn, causes resonant wavelength shift. After zeolite coating, the resonant wavelength shifts towards the shorter wavelength (blue shift) and the sensitivity to external refractive index is enhanced. The resonant wavelength of uncoated LPFG is 1563.1 nm in air and 1560.5 nm in water, with the wavelength shift of 2.6 nm, while the resonant wavelength of zeolite coated LPFG is 1543.2 nm in air. Decreases in cladding diameters and ziolite coating can effectively enhance the refractive index sensitivity of LPFG chemical sensor. © 2010 SPIE.