The mixed crystals of ammonium dihydrogen phosphate (ADP) and potassium dihydrogen phosphate (KDP) with different growth morphologies have been prepared and investigated by X-ray diffraction experiments. All the obtained samples crystallize in the tetragonal system with a continuous expansion of unit cell when increasing ammonium content. The different characteristics of potassium and ammonium lead to the deformation of crystal structure, production of internal stress and degradation of the obtained crystal quality. When ammonium or potassium content in the mixed crystal is low, the accumulated internal stress is weaker and the mixed crystals with high quality can be grown. Contrarily, in the intermediate composition, especially around 65 percent ammonium, the strongest internal stress results in the intensive distortion of crystal lattice and consequent cracks within the mixed crystals. The microscopic mechanism of the morphology evolution of the mixed crystals is proposed and calculated in the chemical bond viewpoint and bond valence model, the difference of the average valence electron density between the KDP and ADP dominates their different growth morphologies. (C) 2007 Elsevier B.V. All rights reserved.