Abstract: Caseins contains three major caseins: α s-casein, β-casein, and κ-casein.The three major caseins were separated, and the purity of each peak was assessed using SDS-PAGE.The purity of the three components were 89.50%, 96.51%, 90.99%, respectively.The three components of casein were subjected to hydrolysis with trypsin.The the degree of hydrolysis and particle size distribution of hydrolyzate were analyzed.The particle size distribution was measured by laser particle analyzer.The conclusion showed that in the three casein components, α s-casein hydrolyzate had the highest degree of hydrolysis.With the extension of the hydrolysis time, the particle size of α s-casein hydrolyzate became smaller.The particle size of β-casein hydrolyzate did not change significantly in 6h.The degree of hydrolysis of κ-casein hydrolyzate increased linearly.The particle size of κ-casein hydrolyzate changed significantly in 10min.