A series of deuterated ammonium dihydrogen phosphate (DADP) crystals were grown and their structures were in-vestigated by using powder neutron diffraction method. In the entire composition range, the deuterated level in the crystals is lower compared with the aqueous growth solution. The deuterium segregation coefficient in the crystals decreases with increasing deuterium content of the solution. The deuterium content in the NH+4 group is higher than that in H2PO?4 group. In addition, the variations of lattice parameters are shown here.