This study was performed for analyzing the general composition and the change in the quality of soymilk-derived yogurts manufactured by adding skim milk and whey powder to soymilk heat-treated at $95^{circ}C$/5 min and $120^{circ}C$/10 min, respectively. 1. During the storage of soymilk yogurt, the concentrations of total solids, protein, fat, and lactose slightly decreased, whereas viscosity, content of ash and NPN, and the number of lactic acid bacteria remained unchanged. 2. The pH and titratable acidity changed rapidly in all soymilk yogurts after 3 h of incubation. 3. We found $7.8{ imes}10^8$ lactic acid bacteria in the control sample, $4.7{ imes}10^8$ and $5.02{ imes}10^8$ in soymilk yogurt with skim milk, respectively, and $5.9{ imes}10^8$ and $5.5{ imes}10^8$, respectively in soymilk yogurt with whey powder according to degree of heat treatment with $95^{circ}C$/5 min and $120^{circ}C$/10 min. 4. The viscosity of yogurt samples became lower as the heat treatment increased in temperature and in the length of time. 5. The value of sensory evaluation was relatively high in soymilk yogurt with the added skim milk, which was heat-treated $95^{circ}C$/5 min; however, the value was significantly lower than that of the control sample. 6. Lactose, glucose, and galactose were detected in all samples because lactose is degraded into glucose and galactose within 3 h of inoculation.