The effect of micelle size on heat-induced interaction between β-lactoglobulin and κ-casein was studied. β-lactoglobulin was isolated from fresh bovine milk and various size of casein pellets were obtained by ultracentrifugation at 100,000 × g for 10, 30 and 60 min. The mixture of κ-lactoglobulin and casein pellet were heated at 65, 75, 85 and 95℃ for 30 min, respectively prior to analysing by means of Sepharose 6B column chromatography, ultracentrifugation and polyacrylamide gel electrophoresis. Heating β-lactoglobulin alone (85℃) showed formation of large molecular weight polymer of β-lactoglobulin in Sepharose chromatography. After heating mixtures containing β-lactoglobulin and casein pellet at 85℃ β-lactoglobulin peak decreased while casein pellet peak increased. Electrophoretic analysis also showed interaction of β-lactoglobulin with casein micelle occured during heat treatment. As the heating temperature increase the protein content of the ultracentrifugal supernatant decreased. Smallest protein content was also found in the small micelle(casein pellet 3). These results indicate chat the interaction of β-lactoglobulin and κ-casein was affected micelle size ascribed to variation in the κ-casein content of micelles.