Experiments were conducted to study the effects of formic acid on the quality of the rye silages at the different dry matter content (19.2, 28.6 and 32.2%) and different formic acid addition levels (0.00, 0.24, 0.48 and 0.71%). Rye were taken at vegetative stage (plant height 40cm) on November 29, 1984. Herbage were adjusted dry matter contents by wilting. Materials were ensiled in small polyethelene film bag after addition planned formic acid, and stored under room temperature. The results obtained are as follows: 1. In the visual observation of silage quality by addition of formic acid in unwilted silage show more clear brownish-yellow color and sweet flavor and less acidity compared with untreated formic acid. 2. The pH values of the silage increased by increasing DM content. At lower dry matter contents in materials the pH values decreased with increased formic acid levels. At higher dry matter content silage, however, non significant difference were observed. 3. The formation of total organic acid decreased by increasing DM content in the materials, and there appeared a significant difference (P<01) among formic acid levels. The organic acid contents in silage decreased with higher formic acid levels. 4. The water soluble carbohydrate in silage increased by increasing DM content. And it was also increased (P<01) with increased formic acid addition level. 5. The production of NH_3 - N decreased (P<01) with increasing the addition of formic acid, and decreased by increasing DM content in materials. 6. The in vitro dry matter digestibility of silages showed not difference among dry matter contents of the materials but at 0.71% addition of formic acid in unwilted silages appeared higher (P<05) digestibility compared the others (0.00% and 0.24%).