This experiment was conducted to study the change of flavor components, especially volatile free fatty acids, during earmentation by Str. thermophilus, L. bulgaricus and the associated culture of the two species. The results obtained are summarised as follows: 1. In the production of total volatile fatty acid, it was found that Str. thermophilus was concerned most significantly and that symbiotic activity was shown slightly. 2. In the change of volatile free fatty acid considered as contributing to the balance of flavor substances rather than main flavor components, the change of acetic acid contents was shown distinctively, and especially L. bulgaricus which characteristics a comparatively high-proteolytic activity in milk, produced the acetic acid in the highest amount, and the symbiotic relationship was shown mildly.