The effects of adding essential oils (EO) at different levels (125, 250, 500 mg/l) on rumen fermentation and biohydrogenation were examined in a rumen batch culture study. Treatments were: control without EO (CON), control with anise oil (ANO), cedar wood oil (CWO), cinnamon oil (CNO), eucalyptus oil (EUO), and tea tree oil (TEO). Essential oils, each dissolved in 1 ml of ethanol, were added to the culture flask containing 40 ml of buffer solution, 2 ml of reduction solution, 10 ml of rumen fluid, 25 mg of soybean oil, and 0.5 g of the diet. After 24 h of incubation in a water batch at 39 degrees C, three samples were collected from each flask and analyzed for ammonia-N, volatile fatty acids (VFA), and fatty acids (FA). Expect for CNO, the proportions of acetate, propionate, and acetate to propionate ratios were not affected (P > 0.05) by EO addition. Addition of CWO, CNO, and TEO reduced total VFA concentrations (P < 0.05) regardless of dose level. The ammonia-N concentration was greater in cultures incubated with EO regardless of dose level. Compared with the CON, the concentrations of C18:0 and trans C18: 1 were reduced (P < 0.05) with EO addition regardless of dose level. Compared with the CON, the concentration of linoleic acid was greater (P < 0.05) when EO were added at 500 mg/l. EO tested in this study had no effects on VFA profile but significantly reduced the formation of biohydrogenation products (C18:0 and trans C18:1).