The purpose of this study was to investigate how the addition of rosemary to corn silage affected the quality, fermentation, and in vitro digestion of organic matter. In the study, unaltered silage group constituted the control group, while silages prepared by adding 0.5, 1 and 2% rosemary constituted the experimental groups. After 60 days of ensiling, the pH of the opened silages was determined to be in the range of 3.67-3.72. As the amount of rosemary in the silages increased, the levels of ammonia nitrogen (NH3-N/TN) decreased. Compared to the control group, increases in in-vitro organic matter digestion (IVOMD) and metabolizable energy (ME) values were observed in the experimental groups due to the addition of rosemary. In comparison to the control group, the methane gas (CH4) rate of silages was reduced in the other groups. Meanwhile, the group receiving 2% addition of rosemary had the highest values of acetic acid (AA) and lactic acid (LA) among the supplemented silage groups. But, propionic acid (PA) and butyric acid (BA) were not detected in any of the silage groups. Consequently, it was determined that 1% and 2% of rosemary added to corn silage can enhance the quality of the silage.