A new class of fiber grating with only cladding index modulation is presented and analyzed theoretically. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. The transmission of a mode guided by the core through a cladding index modulated grating when evanescent field coupling occurs is analyzed with couple-mode theory. Lower attenuation and flexible spectral characteristics can be obtained in comparison with traditional fiber core index modulated grating.