Double hexagonal-close-packed (dhcp) crystal structure is a variant of hexagonal-close-packed (hcp). It is always found in phase transformation process at extreme conditions, i. e., high temperature and pressure. In this paper, structural factor of the dhcp was calculated on basis of the electron diffraction theory and the forbidden reflection law was discussed. If l is odd or the condition, 2h+k=3n, l=4n+2 (n a random integer.), is met, the crystal plane (hkl) will be forbidden to reflect. This predicted result is good agreement with the diffraction patterns from an investigation of phase transformations in commercial magnesium alloy.