The caseins were phosphorylated by sodium tripolyphosphate (STP), and then were hydrolyzed by trypsin. The effect of phosphorylation treatment on enzymatic hydrolysis characteristics of casein was investigated with sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDSPAGE), high-performance size exclusion chromatography (HPSEC) and ultraviolet absorption. The results showed that phosphorylation treatment improved the degree of hydrolysis of casein. TheMWof phosphorlysated casein was decreased at different degrees, and it reached the highest degree for 3h. WhatÂs more, compared with the control, the contents of peptide (molecular weight between 20 ku and 35 ku) were significantly increased. However, the ultraviolet absorption spectra of solution did not changed obviously among the phosphorlysated caseins.