The aim of this study was to determine the effect of essential oils (EO) of oregano, ORE (Origanum vulgare); black seed, BSD (Nigella sativa); laurel, LAU (Laurus nobilis); cumin, CUM (Cumminum cyminum); garlic, GAR (Allium sativum); anise, ANI (Pimpinella anisum), and cinnamon, CIN (Cinnamomum verum) on in vitro gas production (IVGP) and IVGP kinetics of barley, wheat straw and soyabean meal. IVGP values were determined by using rumen liquor from three dry Holstein cows. The findings of this study indicate that the effects of EO, doses, and EO x dose interactions were significant. IVGP was decreased by ANI, GAR and ORE, and only CUM increased IVGP. These EO and their different doses or combinations in diets could be used to improve the performance of ruminants. Moreover, EO may act at different levels in energy and protein metabolic pathways, thus their careful selection and combination may be a useful tool to effectively manipulate rumen fermentation.