Measuring content of free amino nitrogen in milk before and after hydrolysis, the method calculates degree of proteolysis. Adding solution of trichloroacetic acid (TCA) into hydrolyzed cow milk sample to make protein deposited. Then, deposited protein is filtered out. Degree of proteolysis is determined by color change of reaction between free amino nitrogen added to TCA after hydrolysis and TNBS. Features are: simple operation, saving time, and effective.