This experiment was conducted to investigate the effects of propionic acid and wilting on fermentation quality of Italian ryegrass silage. Grass material was wilted for a day before ensiling and propionic acid(0. 0, 0. 2. 0. 5 and 0.8 7~ of fresh matter) was applicated at ensiling time. Chemical composition, fermentation acids, pH, microbial population and distribution of the nitrogen contents were evaluated. The results obtained are summarized as follows: 1. The contents of dry matter and water soluble carbohydrate in the silage were increased by wilting, and crude fibre, NDF and ADF were decreased with increasing propionic acid levels. 2. The pH values of the silage increased by increasing DM content, but decreased with increasing propionic acid levels. Lactic acid content lowed in wilted silage, and acetic acid and butyric acid formation were decreased with increasing propionic acid levels. 3. Total nitrogen content in the silage was i n ~ r e a s e d ( ~ < . 05) by addition of propionic acid and wilting, and was the highest in the prewilting-0.8 Q propionic acid applicated silage. The production of NHBN was decreased with propionic acid and was the lowest in the 0.8 % treated silage. 4. The number of total bacteria and yeasts were estimated $10^6~10^7$, $10^2~10^3$ respectively. Moulds number were decreased with increasing propionic acid levels.