Using the X-ray dynamical theory, the reflection coefficients of X-ray double crystal diffraction in Bragg case were derived. The results of the rocking curves calculated for different kind of crystals and different diffractions show that the peak value and the integral intensity of rocking curve for σ polarization are higher than those for π polarization. And the full width at half maximum (FWHM) of σ polarization is wider than that of π polarization. It is pointed out that when the incident X-ray beam is a circle polarized one or/and the crystals A and B are different kind of crystals or/and the diffraction vector of A is different from that of B, the approximative expression of the FWHM of rocking curve for double crystal diffraction, 21/2∞, which was used by many authors, is invalid.