Casein was phosphorylated using sodium tripolyphosphate (STP) under the following optimal conditions: 5% STP, 30 ℃ and pH 9. The degree of phosphorylation reached to the largest (18.18 mg phosphorus/g casein) after treatment for 3 h under above conditions. And then the different levels of phosphorylated casein were hydrolyzed with trypsin. The degree of hydrolysis, the changes of soluble nitrogen, aminonitrogen and peptide nitrogen contents during enzymatic hydrolysis process were studied. The molecular weight distribution (MWD) of hydrolysate was detected by high performance size exclusion chromatography (HPSEC). The results showed that the degree of hydrolysis and amino nitrogen content significantly (P＜0.01) increased after phosphorylated, and obvious (P＜0.05) differences were found between different levels of phosphorylation. In the hydrolysates of casein phosphorylated for 3 h, the soluble nitrogen and peptide nitrogen contents were significantly (P＜0.01) improved. Compared with the control, the retention time of phosphorylated was shorter, and a new peak appeared at 12 min. The content of peptides with molecular weight between 2×104 u and 3.5×104 u increased obviously compared with the control.