Abstract: Background and Objective: Medicinal herbs and aromatic plants could be used to manipulate rumen fermentation. This study aimed to evaluate the effect of adding herbal and aromatic plants at 1, 3, 5, 7% of total ratio DM supplementation of the incubation media of an in vitro rumen model. Materials and Methods: About 400±4 mg of feed sample (roughage and concentrate ratio of 45:55%) with each level, weighted into 125 mL glass bottles (6 bottles for each treatment), rumen fluid injected into these bottles and incubated at 39°C, after 24 h incubation digestibility of dry matter (IVDMD) and organic matter digestibility (IVOMD), total gas production (TG) and metabolic energy (ME) were studied. Results: The differences among plants, added at different levels, were significant. Significant differences were also observed between highest level added compared with control in ammonia (NH3) and volatile fatty acids (VFA), IVDMD and IVOMD compared with control. Total gas (TG) was significantly higher at level 7%, especially with added Melissa compared with other plants. Metabolic energy (ME), was significantly higher in all treatments compared with control. Conclusion: It may be concluded that addition of different medicinal and aromatic herbal plants Caraway (Carum carvi), fennel (Foeniculum vulgare) and Melissa (Melissa officinali)), especially at highest levels tested has a great potential in manipulating rumen fermentation, which may be of benefit when applied in ruminant nutrition.