Plant extracts may be potential replacements for antibiotic feed additives used in livestock production and they are considered safer. Effects of essential oils (EO) extracted from rosemary (Rosimarinus officcinalis; REO) leaves and clove (Syzgium aromaticum; COE) flower on in vitro rumen fermentation parameters including methane production were examined. A corn silage-based TMR (0.5 g; CP 16.6%; NDF 35.9%) was treated with CEO or REO at rates of 0 (Control), 10 (Low), 20 (Med) and 30 (High) μL/50 mL of rumen fluid-buffer inoculum (ratio 1:2) and with monensin (0.6 mg/50 mL). Each suspension was incubated in a 120-mL gas-tight culture bottle in triplicate at 39oC for 24 h in each of 2 runs. Fermentation parameters, gas and methane production, in vitro DM digestibility (DMD), and fermentation efficiency (FE; DMD g kg-1/gas volume) were measured. Data for each EO were separately analyzed with the Glimmix procedure of SAS. The DMD was reduced (P < 0.05) by monensin, Med REO and High CEO (526, 531 and 513 vs. 555 g/kg). Gas volume (mL/g DM) was increased (P < 0.05) by Low REO or CEO (84.5 vs. 92.4 and 96.2) and decreased (P < 0.05) by their High rates or monensin (76.7, 63.6 and 66.8), respectively. The FE was increased (P < 0.05) by High REO or CEO or monensin (7.27, 8.69 and 7.92 vs. 6.58) and reduced (P < 0.05) by Low REO (5.85), respectively. Methane production (mg/g DM digested) was reduced (P < 0.05) by High REO and CEO and monensin (8.65, 8.02, and 7.10 vs. 11.44). The pH was increased (P < 0.05) by monensin (5.75 vs. 5.66) but not by EO. Ammonia-N and VFA concentrations were unaffected by treatment except that monensin reduced (P < 0.05) acetate concentration and increased valerate concentration. Also, monensin and Med and high CEO increased (P < 0.05) butyrate concentration. High doses of essential oils from clove and rosemary decreased methane production and increased fermentation efficiency in a manner comparable to monensin