Emp loying the coup led－mode formalism, the forward and backward wave expressions are deduced under the boundary conditions. Taking up the uniformed fiber grating, it is found that the energy distribution inner the grating is oscillatory. The distributions of forward and backward waves power decay along the grating direction, yet their interference energy one is oscillatorywith the period as same as that of the grating in the forbidden－gap. Beside and outerside the forbidden－gap, the distributions of the forward and backward waves energy are also oscillatory. It is the same to their interference one, but different in the period that their interference period is equal to the grating period, and interference envelope period is equal to the forward and backward ones,which varieswith incidentwavelength thatmore is the departure between incidentwavelength and Bragg one, less is the period. There ismaximal energy amp litude of forward wave beside the forbidden－gap. The energy amp litude becomes biggerwith the longer grating.