This experiment was conducted to study the changes of physico-chemical properties and their contents of casein micelles by heat treatment at various temperatures and times. Fresh or heated casein micelles were fractionated into different sizes by various ultra-centrifugation times and the relative amount of protein of each fraction to the total protein was determined. Some properties of these fractions were also tested by rennet action. Heat-induced changes of casein micelles were observed by polyacrylamide gel electrophoresis. The results were summarized as follows; 1. When skimmilk was ultracentrifuged at 100,000xg for 60 minutes, the proportions of the casein micelles sedimented, serum casein and whey protein to the total protein were 69.97%, 9.86% and 20.47%, respectively. By the heat treatment at 95℃ for 10 minutes, the proportions of these fractions to the total protein were changed to 76.13%o, 20.31% and 3.56%, respectively. 2. When skimmilk was ultracentrifuged at 100,000xg for 10, 30 and 60 minutes, the proportions of pellet 1, 2, 3 and serum casein to the total casein were 27.17%, 45.52%, 14.91% and 12.40%, respectively. In case of heat treatment at 95℃ for 10 minutes, the proportions of these fractions to the total casein were changed to 17.42%, 28.64%, 32.88% and 21.06%, respectively. 3. Non-protein nitrogen soluble in 12% TCA released from ultra-centrifugal serum casein (100,000xg, 60 min) by rennet action was decreased with increase of heating time and temperature. 4. As the results of gel electrophoresis, a new band appeared between α_s-casein and β-casein and its density was increased gradually with the heat treatment at 75℃ or higher temperature.