More and more efforts are endeavored in the ultra-sensitive quantification of peptides and proteins, because many important peptides and proteins are present at ultra-low levels and only the quantity of them or the changes in their abundance really reflects the status and changes of a biological system.We herein reported a novel ultra-sensitive method for the simultaneous quantification of three β-casomorphins based on the Eu3+ diethylenetriamine-N,N,N,N",N"-pentaacetic acid labeling and capillary electrophoresis with on-line inductively coupled plasma mass spectrometry detection (CE-ICP-MS).The europium-tagged β-casomorphins were effectively separated by capillary electrophoresis and were sensitively determined by inductively coupled plasma mass spectrometry.