This experiment was carried out to investigate the effects of harvesting date and additives on the quality of fall sown oat(Avena sativa L.) silage at the department of animal resources science, college of life resources science, Woosuk University, Wanju &om August, 1997 to March, 1998. The experiment was arranged in a split plot design with three replications. The main plots consisted of the harvesting date such as 15 Oct., 23 Oct. and 2 Nov. and different additives as subplots(control, wheat bran, formic acid, lactic acid bacteria, and wilting). The results otained are summarized as follow : The fiesh and dry matter yield of oat were increased by harvesting date delayed, then fiesh and dry matter yield at 2 Nov. were highest as 36,733 and 7,29Okg/ha, respectively. The dry matter content of oat silage was increased significantly by harvesting date delayed(P< 0.05), and DM content of wheat bran and wilting treatment was high at every harvest date. The DM loss was lower in LAB, formic acid and wheat bran treatment, but on significant difference was found among harvesting date(P< 0.05). Average pH of silage harvested at 23 Oct. was lowest as 4.06, and the quality(CP, NDF, ADF, and IVMDM) of oat silage was decreased by harvesting date delayed. The content of acetic acid was no difference among harvesting date, but butyric acid was decreased and lactic acid was increased at 23 Oct. harvest. The result of this study indicate that oat harvested at booting-heading stage of maturity with wilting wheat bran, and LAB could be recommended as producing high quality of oat silage.