The effects of adding essential oils (EO) on rumen fermentation and methane production were examined. The aim of experiment one was to screen the effects of four different EO (clove oil (CLO), white thyme oil (WTO), citronella oil (CTO) and anise oil (ANO)) at 500 mg/L of culture fluid on methane production under in vitro conditions. Rumen contents collected from a cannulated Holstein dairy cow was used in a 24-hour batch culture experiment. Treatments were a control (CON) or CON plus EO at 500 mg/L. Results showed that all EOs, except CTO, decreased (p <=.05) methane production. The aim of experiment two was to test the effects of three different dose levels of CLO, WTO, and ANO on methane production and fermentation in 24-h batch culture experiments. Treatments were CON or CON plus EO supplemented at 125, 250, and 500 mg/L. Relative to CON, methane production decreased (p <=.05) with the three EO at the 500 mg/L dose. At the 250 mg/L dose, ANO and CLO decreased (p <=.05) methane production and at the 125 mg/L dose, only CLO decreased methane production. Relative to CON, total VFA concentration declined (p <=.05) in cultures incubated with WTO and with ANO at 500 mg/L dose. Relative to CON, the addition of CLO, WTO and ANO at 500 mg/L decreased (p <=.05) dry matter (DM) digestibility. In conclusion, our results showed that EO effects on methane production depend on EO source and dose level. Although the addition of ANO and WTO at the high doses resulted in lower methane production, they had negatively impacted on rumen microbial fermentation. Clove oil on the other hand reduced methane production without negatively impacting rumen fermentation.