Abstract: A series of extraction equilibrium experiment for L - phenylalanine withdi(2 - ethylhexyl) -phosphoric acid (D2EHPA) dissolved in n - octane was carried out. The effects of L- phenylalanine concentraction,D2EHPA concentraction and pH on extraction distribution ratio were discussed in detail. The infrared spectrogram of loaded organic phase illustrated that pH condition had little effect on the structure of the complex. There are proton - transfer reaction and ion - exchange reaction in the extraction . One L - phenylalanine is extracted by forming a complex with two dimeric D2EHPA molecules. An expression of the equilibrium distribution was proposed.