The resonance equation of the sampled fiber Bragg gratings is derived through the Fourier′s transform and the coupledmode theory, which determine the wavelengths of the resonance peaks. The equation is related to both of the grating period and the sampled period, which have no relation to the duty ratio, the length of the gratings and the coupling coefficient. Both of the result calculated from the resonance equation and the transmission matrix are compared and they are approximately equal. The interval of the neighboring peaks can be derived conveniently by the resonance equation. The result is useful to the investigation and design and the fabrication of the sampled fiber Bragg gratings.