This experiment was conducted to evaluated the effect of maturity at harvest on the changes in quality of round baled rye silage at forage experimental field of Grassland and Forage Crops Division, National Livestock Research Institute, RDA, Suwon in 1998. The experimental design was a split-plot design with three replications. The main plots were three different harvest stages : boot, heading and flowering stages, and the subplots were days after ensiling : 1, 2, 3, 5, 10, 30, 45, and 60 days. The wilting period of boot, heading and flowering stages were 1, 0.5 and 0.5 days, respectively. The final pH of rye silage was higher in the order of flowering, boot and heading stages. And pH of flowering stage began to change at early fermentation period, but that of boot and heading stages was delayed 1~2 days. Ammonia-N content of boot stage was highest. and that was increased as fermentation progressed. But Ammonia-N of heading stage was decreased to 30 days. then that was increased after 45 days fermentation. Among fermentation periods, inside temperature of deep place was not affected by external temperature. And that of deep place was increased to 3$0^{circ}C$ at early fermentation. then decreased as fermentation progressed. However surface temperature was affected by external temperature after 10 days. Acetic acid content was not changed with 5 days by harvest stages, but that of boot stage was increased after 10 days. Butyric acid of boot stage was increased after 5 days. but that of heading stage was increased after 10 days. However lactic acid was increased from 1~2% to 6~8%. Lactic acid bacteria (LAB) of heading and flowering stages were highest at 5 days fermentation, and that of boot stage was highest at 10 days fermentation. The results of this study indicate that fermentation of round baled rye silage occur within 5 days. Therefore, any modification should be applied with an 5 days for high quality of round baled rye silage.