Calcium phosphates (CaP), sparingly soluble phosphoric acid salts, have the main role in biological and pathological mineralization and in industrial processes. Today, 13 phases of CaP are known. Calcium hydrogenphosphate dihydrate (DCPD) is one of the major calcium phosphates minerals which can be found in dental caries, kidney stones and used in the cosmetics and pharmaceutical industries. Because of its crystal structure and morphology, DCPD represents a good model system for studying additives and crystals interactions. The aim of this research was to determine optimal conditions of synthesis of DCPD seed crystals in precipitation system Ca(OH)2-H3PO4 and investigate the influence of different amount of seed crystals on calcium phosphates crystal growth and formation in metastable solution in conditions similar to physiological (pH 7,4, temperature 25 °C). DCPD was prepared by rapid mixing of calcium hydroxide and phosphoric acid solutions. Systems were magneticaly stireed for 1 hour at 25 ± 0.1 °C. Crystal growth and transformation of DCPD seed crystals in metastable solution (c(CaCl2) = c(Na2HPO4) = 4,25 mmol dm^-3, c(NaCl) = 0,15 mol dm^-3, pH 7,4) were followed potentiometrically. Precipitates were taken after different times determined on pH curves and characterized by IR spectroscopy, X-ray diffraction and scanning electron microscopy. Obtained results have shown that optimal reactant concentration for DCPD sysnthesis are c(Ca(OH)2) = c(H3PO4) = 0,01 mol dm^-3. Mass of 1 mg of DCPD seed crystals is optimal for following kinetics of DCPD transformation. Addition of DCPD seed crystals in metastable solution initiates precipitation of calcium deficient hydroxyapatite.