The objective of this study was to determine the effect of inclusion of oregano oil (OrO), peppermint oil (PO) and orange oil (OO) in the ratio of 0, 400, 800 and 1200 mg/lt into rumen fluid on the gas production, organic matter digestibility (OMD) and metabolizable energy (ME), rumen fermentation characteristics and microbial protein production using the in vitro gas production technique. The inclusion of OrO, PO and OO in a different level significantly (P < 0.05; P < 0.01) decreased the in vitro gas production, OMD and ME values. The inclusion of essential oil decreased the total volatile fatty acids (VFA), acetic acid, propionic acid, butyric acid, acetic acid/propionic acid ratio and ammonia level whereas the inclusion increased the rumen pH. In addition, the inclusion of oregano oil significantly (P < 0.05; P < 0.01) decreased carbon dioxide (CO2), methane (CH4) and microbial protein production (MPP). The OrO was the most effective essential oil when all parameters were taken into consideration, followed by PO and OO. The inclusion of OrO in the ratio of 1200 mg/lt has been found to be the most effective doses and a negative impact on the data obtained. In conclusion, it can be said that low doses of essential oils should be used in ruminant nutrition but high doses of essential oils may decrease the rumen function and feed efficiency.