An expression for the diffraction intensity from a crystal composed of antiphase domains is derived. An early intensity formula, in which the influence of the antiphase domain boundaries on intensity is neglected, is altered by adding two parameters; one is the number density of the domain boundaries and the other the phase angle of the structure factor. The newly derived formula is successfully applied to observed diffraction intensities from a triglycine sulfate crystal in multi- and single-domain states, which are generated by controlling the strength of an external electric field. We propose a new method to estimate the number density of the domain boundaries.