In order to know the influences of heat treatment of yoghurt on pH, $eta$-galactosidase and viable cells, yoghurt sample was made by general method with Lactobacillus bulgaricus and Streptococcus thermophilus, and the changes in pH, $eta$-galactosidase-activity and viable cell-count were determined during heating at 55$^{circ}C$ and 7$0^{circ}C$. The pH of yoghurt was not changed when the yoghurt was heated at 7$0^{circ}C$, but at 55$^{circ}C$ it decreased slightly. The stability of $eta$-galactosidase was not affected markedly by heat treatment at 55$^{circ}C$, but was rapidly inactivated at 7$0^{circ}C$. The heat treatment of yoghurt at 55$^{circ}C$ had the halb of viable cell in 1 hour, but the heat treatment at 7$0^{circ}C$ had considerable effect on viable cell in 5 minutes.