The objective was to examine the effects of essential oils (EO) from Syzygium aromaticum (clove), Cinnamomum zeylanicum nees (cinnamon), Rosmarinus officinalis (rosemary), Zingiber officinale (ginger), and Cymbopogon citratus (lemon grass) on in vitro dry matter digestibility (IVDMD), ruminal fermentation and methane production using a TMR as substrate. A corn silage-based TMR was treated with each EO at doses of 0 (CON), 200 (Low), 400 (Med) and 600 (High) mg/L of buffered rumen fluid and with Monensin (MON) at 12 mg/kg substrate DM (positive control). A randomized complete block design with 3 replicates per treatment and 3 runs (blocks) was used to determine the effects of EO and MON. Data for each essential oil were separately analyzed with the GLIMMIX procedure of SAS and treatment effects were estimated using multiple comparisons. Each treated substrate was incubated in a 120-mL gas-tight culture bottle at 39°C for 24 h. The EO had no effect on IVDMD when compared with the CON. However, compared with MON, IVDMD was increased (P < 0.05) by the low dose of rosemary and ginger. Gas volume was reduced (P < 0.05) by the low dose of rosemary and low and high doses of cinnamon, compared with CON. All EOs except lemon grass reduced (P < 0.05) methane production at all doses when compared with the CON. The high dose of rosemary increased (P < 0.05) pH compared with CON and MON. Lactate content was significantly higher (P < 0.05) in med dose of cinnamon. Acetate concentration was reduced (P < 0.05) by medium and high doses of clove and cinnamon while isobutyrate concentration was increased with all doses of clove and ginger, compared with CON and MON. The high dose of lemon grass reduced (P < 0.05) propionate concentration compared with the MON. The high dose of cinnamon increased (P < 0.05) butyrate concentration compared with the CON and MON. In conclusion, compared with the CON, all doses of EOs reduced in vitro methane production without reducing digestibility whereas MON had no effect on digestibility, methane production or gas production.