The transmission characteristics of the filter composed of a Sagnac loop and fiber Bragg gratings are studied in detail. The analytical results are given for the uniformed fiber grating. It was found that the transmission spectrum of the filter is the cosine function modulated reflection spectrum of the fiber grating. When the fiber grating is “strong” coupling, a multi-passband filter can be constructed. When the fiber grating is “weak” coupling, a single-passband filter can be constructed. These filters can be used in modern optical fiber communications.