Capillary electrophoresis-mass spectrometry (CE-MS) has been developed as a powerful tool in the analysis of charged compounds. To simultaneously analyze free amino acids, an electrolyte with low pH was used to positively charge all of the amino acids. In this condition, all protonated amino acids migrated toward the cathode in CE and then were sensitively and selectively detected by MS. This method is simple, rapid, and selective and could readily be applied to the analysis of free amino acids in various samples. In this chapter, the detailed procedure to analyze amino acids using CE-tandem mass spectrometry (MS/MS) is described.