The aim of this study was to investigate the in vitro effects of the addition of three essential oils, mainly cuminaldehyde, eugenol, and thymol to alfalfa (Medicago saliva L.) silage on gas production, digestibility, and metabolic energy. The essential oils were added at 0 ppm (controls) and 100 ppm (group 1), 200 ppm (group 2), and 300 ppm (group 3) with three replications. The cumulative amount of gas resulting from the addition of the three essential oils at the 96th h of incubation was significantly lower than that of the control group (P < 0.05). The cumulative amount of gas production at the 96th h of incubation with thymol ranged from 184.63 milt; to 217.38 ml./g. In conclusion, this study showed that the addition of different amounts of cuminaldehyde, eugenol, and thymol to alfalfa silage reduced the amount of in vitro gas production. Therefore, these treatments can potentially reduce environmental pollution from ruminal digestion. However, the use of increasing levels of essential oils reduced the amounts of digestible organic matter and metabolic energy. Additional studies are necessary to reveal the effects of these essential oils on in vivo ruminal fermentation by measuring changes in feed consumption and productivity.