Abstract: To investigate the mechanism of binding Ca~(2+) and the effect of inhibiting calcium phosphate crystals formation of isolated phosphopeptide( IPP) derived from carp egg peptide( CEP). The Ca~(2+) binding sites of IPP were determined by mass spectrometry( MS) and Fourier transform infrared spectroscopy( FTIR).The change of its space structure after binding Ca~(2+) was studied through FTIR and circular dichroism spectroscopy( CD).The effect of IPP inhibiting calcium phosphate crystal formation was observed by electron microscope. The results showed that phosphate had the priority to binding Ca~(2+) .One mole of IPP could bind four moles of Ca~(2+) and carboxyl group could not bind calcium.Regardless of the presence of Ca~(2+) ,IPP was present in the state of unorded structure in solution under physiological conditions,without any ordered secondary structures.In the supersaturated solution of hydroxylapatite( HAP),IPP could compete with phosphate to bind Ca~(2+) and adsorb to the surfaces of crystal nucleus,which inhibited the nuleation of calcium phosphate and its aggregation to crystal.