Li, Y. L., Li, C., Beauchemin, K. A. and Yang, W. Z. 2013. Effects of a commercial blend of essential oils and monensin in a high-grain diet containing wheat distillers' grains on in vitro fermentation. Can. J. Anim. Sci. 93: 387-398. Our objective was to evaluate in vitro effects of a commercial blend of essential oils (BEO) versus monensin (MON) on fermentation of a high-grain diet containing wheat distillers' dried grains with solubles (DDGS). Two experiments were conducted. The first experiment was designed as a short-term batch culture to determine the optimum dose of BEO (0, 45, 90 or 180 mg kg^-1 DM) on fermentation variables. The second experiment was a completely randomized block design with a 2×2 factorial arrangement of treatments using the rumen simulation technique. A high-grain dietary substrate containing wheat DDGS was supplemented with 0 or 28 mg MON kg^-1 DM combined with 0 and 90 mg BEO kg^-1 DM. Monensin did not affect the total volatile fatty acid (VFA) concentration or the molar proportions of individual VFA, but reduced CH₄ production (mL L^-1 gas; P=0.01) and disappearance of DM (P=0.04). Supplementation of BEO did not affect VFA concentration, but decreased (P=0.01) the molar proportion of acetate and reduced (P=0.02) the acetate to propionate concentration ratio and CH₄ production (mL g^-1 digested organic matter). The BEO increased (P=0.01) disappearance of neutral detergent fiber without affecting disappearance of other nutrients. These results indicate that supplementation of high-grain dietary substrate containing wheat DDGS with BEO improved fermentation pattern by increasing propionate concentration, reducing CH₄ production, and increasing fiber digestibility. This commercial BEO could be a potential substitute for MON for beef cattle fed high-grain diets containing DDGS.